If you want the EZ-mode leveling route, it goes:
1-12 Newbie zone+adjacent dungeon
12-19 Oasis
19-25 Unrest
25-35 Mistmoore
35-55 City of Mist
55-60 Chardok AE (buy it or roll a class that can do it).
There you go, cookie-cutter/fast-lane/snoresville grouping. You can do any of these zones at ±2 level variance pretty easy, but you're better moving on sooner than later, assuming you're not a hybrid/the groups will take you.
Other popular zones with groups (usually):
Highkeep Hold: 25-33
the Overthere: 20-42
Lake of Ill Omen: 20-30, Bloodgills 40-45 (I think? never done bloodgills)
Warslik's Woods: 20-30
South Karana: 20-34
Karnors: 50-60
Awesome BYOG zones:
Guk: 20-30
Lower Guk: 30-50
SolA: 20-40
SolB: 35-late 50s
Relevant Links:
http://wiki.project1999.com/Recommen...s_and_ZEM_List
http://wiki.project1999.com/Per-Level_Hunting_Guide