Hallo and welcome! i see a lot of people asking in game about "what pack should i buy" "which should i get first".
ill try and keep this kind of short, for more in depth reviews, check out sirgog's thread.
this thread focuses more on maximum benefits vs. cost, note i am somewhat biased, so its not gospel here.
also, remember, be patient! if you are not immediatly going to run the pack, do not buy it! try and buy packs on sale if you can
To start off, if you buy nothing else buy: (in this order)
Vale of Twilight- this is the one absolute must in the game. Not only can it get your first life comfortably to 20 from 15 or so, it also includes the most run raid in the game, as well as the most sought after equipment in the game (see greensteel). The quests are also fun and varied.
Devils of Shavarath (often referred to as Amrath)- This is the true endgame, and contains the hardest raid in the game. The quests all have different mechanics, and since the wartorn battlefield hasn't been spammed in your face, its actually quite a cool explorer. Buy this after vale of twilight.
If you like ddo a lot, then buy: (in this order)
Gianthold- Great mid level pack with some wothwhile gear. A ton of quests, the flagging ones are quite interesting, and the non-flagging ones are quite good for xp zerging if you feel like it.
Vault of Night- Epics, quests are especially varied, and especially interesting raid. Very solid pack for lower level and capped players, eVault of Night is my personal favorite raid. Good loot in epics, some of the best at-level loot in non-epic
Sands of Menechtaurun- this might be more important than Vault of night to some, has different mechanics in each quest if the same environment. Very difficult (good thing) on epic, has astounding epic loot, which is also the hardest to acquire in the game. Good normal loot too, as well as the famed bloodstone (though those are tradeable).
Note: if you do not care about epics overmuch, and already have gianthold, you probably dont want to buy this
If you really like ddo, and feel like investing more-
Necropolis part IV- solid mid-late level pack, more important for a caster (above sands), pretty amazing caster loot as well as varied quests.
Reaver's Reach- solid pack, good easier "endgame" content, more important for melee (above sands), dragontouched armor crafting improvements make it very desirable for many melees
(this belongs in the below collection, but i feel i must address this) Delera's- do NOT put this before other packs if you are new/ want to save money. I see this as highly recommended, but that is merely because the xp is excellent. Quest-wise, the quests are basically all "kill lots of undead". Very good for xp, and some nice end rewards (voice of the master especially), but in character progression, you can level past it without much trouble, and you arent missing much in terms of interesting content.
other mid- high level packs note that you may derive more personal enjoyment from these than other packs, if something looks really interesting to you, go for it. Other packs with epic like red fens and assault on stormreach might be a good choice, also, phiarlan carnival has interesting quests and nice "beginners" epics, so that can also be considered. The Inspired Quarter is a pack that i liked very much personally, with some ok loot, good xp in group also. The new Harbinger of Madness pack is also a personal favorite of mine for atmosphere.
Necropolis part III, combined with frustrating quests, and the fact that gianthold pretty much nullifies any need to go here, its just not really worth it.
Ruins of Threnal- frustrating flagging mechanics, boring quests, high tp cost. its "ok" i suppose, but it just isnt worth the tp cost at all.
Any lower level (1-6) packs-with the exception of the phiarlan carnival, they are mostly pretty dull/have no epic difficulty, and you dont need more xp for these levels anyway. Not "bad", just "not worth it"
it should also be noted that quest are much, much more important than classes and extra character slots. they provide actual content.