This is not going to be a thoroughly scrutinized guide. I'm just going to go through what I consider to be the fastest way to hit 16 having good experience in doing so. I have no frame of reference for people who cannot send their lowbies cash in the mail so this isn't a new player guide so much as a "level up your lowbie with money from your level 16 guide."
Preface: If your new character is melee send him a stack of cure light potions. If your character is a cleric, send him a few wands. Common sense stuff. Always be looking on the Auction House for cheap stat items that have no additional mods. Like if you see a +1 or +2 con belt with nothing else on it, buy it and save it for your potential new toons. A failsafe way to go about assessing what to buy is to have a plan and follow it according to item slot.
Tell yourself, "Ok on my lowbies i'm always going to equip con to my belt, str to my gauntlet, dex to my glove, striding and featherfall to my ring or boot, wisdom to my neck or ring, and swap underwater action with a ring, etc."
this way you will develop a power level set you can keep trading to new toons as your lowbies level out of the stuff you have.
Level 1: Just do the quests in the opening sequence all on normal difficulty solo. It's actually easy as hell even for casters + melee characters to just plow through using whatever weapon you have to club **** to death all the way up to making it to stormreach. I would suggest just doing normal difficulty and not doing solo diff because solo diff is useless.
As soon as you reach stormreach either repeat the mayor's quest in the opening sequence on hard (also soloable) for set items or walk into any level 2 quest in storm reach and solo it until level 2.
total time: you should be done with the preliminary quests within 20 minutes if you just keep running. make sure to kill stuff in the explorable zone as you go for bonus exp. 200 kills is probably the most efficient exp and what comes naturally in running these quests.
Level 2: Waterworks. As soon as you're level 2 grab your friend or even a level 1 to haul through waterworks on normal. Opening the marketplace is the most important thing at level 2. At this point I like to try to find a "harbor elite" group to knock out as much favor as I can while I can get maximum benefit from doing so. that way it doesn't seem like a massive waste of time when you're level 16 running elite level 3 quests.
Total time: 1 hour since creation of character you should be done with waterworks and about level 3.
Side Note:Unfortunately (or thankfully, whichever you see it as) you will be level 3 before you have a chance to do all of the level 2 quests. at level 3 you want to run a couple of annoying quests like redfang. don't be afraid to START a group. Some people make the mistake of waiting to spot a group recruiting for the quest they want to do next and that doesn't happen very often. You will find that people come knocking very quickly when you start a group. As a rule I start the group and search the "who" panel for people within 1 or 2 levels in either direction of me and send them all a prefab tell about the quest i want to do next and ask if they are interested. don't bother asking people who have the "grouped" icon next to their names.
the classes you usually want are wizard, cleric, tank, rogue. if your group has all of those classes it is almost guaranteed to be a cake walk. however when you're level 3 and pimping yourself out with your 16 you can generally use your bank to make yourself seem better than you really are. this is handy because you can lay down the ownage on any quest without any help usually. the trick is having a good memory about the nuances about every quest you intend on doing. if you don't know a quest, that is when you really need to make sure you have a party.
Level 3: At level 3 only recruit level 4's and 5's. Level 2' swill almost certainly still be stuck in the harbor. Run STK on normal, hard, then elite. You will then be level 4. You might try redfang and a couple of other market quests in between.
Total time: 2-2.5 Hours since creation. the main thing is making sure you have either a cleric or a caster to kill the boss at the end of stk who is not "new to the game." i use parentheses to eliminate the dreaded "N" word. also run the explorable zone in harbor at least once.
Level 4: Level 4 is a funny level. If you are a good player who has given yourself the world with your alt, you will easily get into stormcleave and gwylan's stand groups. Rogues need to focus on having pick locks, search, and disable. give yourself + tools and items to swap for search and disable. make sure you invest in those skills more than your melee presense at this level all the way up to level 12. Clerics need to bring loads of wands. Casters need to focus on only casting when it is most beneficial. If you see a group of monsters, web. If you see an ogre or giant, cast a will save spell like command (if cleric) or fear. know your enemies and plan your sp usage as though every move you make is choreographed. this is something that will be useful throughout the entire game. melee, well, you know what to do. get a transmuter or adamantine weapon as soon as possible.
Total time: 3-3.5 hours since creation. it depends on who you know and what group you get in. make sure people aren't idiots in gwylans and keep track of crates even if you aren't carrying them to ensure a quick completion.
don't wait for people to dink around with traps and drag **** through fire. even as an 8 str cleric you should be able to jump past the traps and just take everything out with a +2 heavy blunt simple weapon if you hit yourself with bulls strength pots. if people are complaining about you going too fast, you're doing a good job at making things go as fast as possible.
at this point as a pally, cleric, ranger, etc, you need to start thinking about who you are going to heal and when. unfortunately not all people were created with equal intelligence. the ranger with 2 hp who insists on dual wielding and taking ridiculous dmg? let him die. the tank who does great dps and is sturdy? heal him. the wizard who tries to tank because he took a level of fighter? let him die. the wizard who acts as a real caster? MAKE SURE HE LIVES.
you get the idea.
You should still be running stormcleave and gwylans as well as adding tangleroot runs. tangleroot is good to hold off on till 5 because you want the explorable zone exp to work for you toward 6. at this stage tangleroot can be done on both norm and hard for the bonus exp. Remember if you are standing still, you are not getting exp. So keep it moving and ignore people who complain about running through dungeons.
Total time to complete level 5: 4-5 hours since character creation.
Ok, drop everything and run grey moon and co6. Just doing this one series of quests will put you at 3.75 enhancement points into level 6. after that, run delera's. normal if you can't find a wizard with halt undead. hard or elite if you can find one and the average level of player is 7.
Total time to complete level 6: 6 hours since character creation. (if you are not on mark with 6 hours you are moving way too slowly. bring haste pots and keep it moving.)
make sure you have a deathblock robe or shield and appropriate fortification for your level. deathblock robe is valid even for tanks because you can switch to it for beholders which do not care about your ac.
You now get a pile of new level restricted gear and it's all highly needed. Run the depths quests in house d. normal, hard, elite. run deleras, stormcleave, tangleroot, and gwylans elite 2-3 times each. You will be level 8.
total time to complete level 7: 7.5-8.5 hours since character creation. don't worry if level 7 takes a long time. it sometimes is a pain to find groups suiting your best interest of speed running. be careful not to make a bad name for yourself as well. at level 7 you will be dealing with level 9's who you will continue to see with this character for the rest of the game. people lower than 7 probably don't matter and/or they will forget if you left them in the dust by the time they hit 16 because most people take months to hit 16. you can also just be diplomatic and sweet talk those b***ches into believing you care about team work after the fact.
rerun co6 on hard/elite, focus on house p and k quests that are level 8. you want to get house p buffs and pendent of time as well as extra storage quickly. make sure your lower level necropolis quests are done on normal. start thinking about the harder ones.
Total time to complete level 8: 8-9 hours since creation.
You might have noticed that you have burned out a lot of your level 8 quests. Good! Move on to level 11 type quests. You can easily get prison of planes groups together as any character class at level 9-10. Big time exp to be had. stick to level 12 stuff as a rule though to make sure you suck those dry before hitting anything you get exp penalties on. Run tempest's spine at least once at 9. also aim to complete some of the harder necropolis quests.
Total time to complete lvl 9: 10 hours since character creation.
Think about maximizing loot while getting exp by running quests like xorian cypher, tempest's spine, and von 1-4.
repeat quests like the second chapter of delera's, run thernal ruins if you already haven't for p favor, get giant hold relics, go to mechatrun desert and get demon queen ready. all of the desert quests are easy exp except trial by fire.
tempest's spine elite is a must.
total time to complete level 10: 12 hours since character creation.
keep running giant hold quests and try a few level 12ish quests on hard mode. you should still be able to tap a few high yield runs that are lower than you for exp. run von 6 the dragon raid and get ready for titan.
Total time to complete level 11: 14-16 hours since character creation depending on if you get raid ready or go for exp. either way you will have to get raid ready so kill 2 birds with one stone and spend the extra time.
make sure you run 3 raids by the end of level 12. titan, demon queen, and dragon from von. use the orchard and gianthold for exp and loot. acquire tapestries and make an appropriate hat (most likely the minos hat.) you should have the majority of your relics from giant hold by now and seek to complete madstone on normal mode once along with the other mandatory gh quests. it will take a while to complete level 12 even if you constantly join elite gh quests. all necropolis quests must be complete on normal at least.
total time to complete level 12: 16-17 hours since character creation.
there are some things everyone must have by this point in the game.
For melee and ranged: a good banishing weapon, a vorpal, a disruptor, hopefully a smiter, wounding/puncturing, and start looking for greater bane items. ranged toons need to make sure they have solid house D favor to take advantage of the missile vendor.
casters and clerics need maximize, empower, extend, and quicken by this level. also be looking for potency items. greater potency is good and much more level friendly. you also will need significant spell penetration for devils and outsiders in order to banish, destruct, finger, etc. so take the enhancements for spell pen and start gathering blue scales for your giant hold armor.
p.s. it's always possible to be a slouch and still reach 16 but these tips are for people who want to do it quickly and be efficient at killing/healing.
level 13 & level 14:
for these levels you should be running orchard quests and thinking about making your way out to meridia by 14. coalescence chamber is a good run and can be soloed by most people who are pimping their lowbies. aim for groups but always be thinking about getting banishing weapons etc for the vale. the vale is going to be a gold mine for those who can ravage it with the right tools for this level range. grab vale groups and kill the named mobs thinking about what you will want to craft for green steel. casters should consider a weapon and a piece of armor such as goggles.
total time to reach level 15: approx 22 hours of play since character creation. (make sure you're sleeping and going to work still ;P )
level 15 & 16:
once you hit 15 you are basically set. (for this mod at least) you just want to keep running the shroud raid and getting ingredients, keep running reaver's fate for the exp and items, try to get groups for demon queen and titan. run anything over level 14 that you see advertised and try to tap out the vale quests. you won't be able to get them very low because you will be capped before long.
total time to reach level 16: approx 26 hours of play since character creation.