I got all the explorers and whatever slayers you get while getting the explorers for Kings Forests, Drow City, and Demonweb (I didn't do much exploring in the Underdark).
I also did all the Eveningstar epic challenges one time each... That's worth 300k or so.
I did some of the Cannith epic challenges one time each... Another 200k-300k.
I ran the VON series 1-4 on epic twice (once hard, once normal). I ran the House P epics once on hard. I did eChrono twice. Oh, and I ran the U15 quests on epic normal once.
I didn't touch the Sentinals epics, the Red Fens epics, or the Sands epics, or VON 5-6, or the new raid.
Soon enough we'll have Gianthold epics to add to the list.
There's a ton of exp to get you from 20-25.