[Tallenn]

Some of my favorites:
15-20 or so Highpass
25-35 High Keep

Kunark was out by the time I got to 35 on live, so I started going there mostly, after that.

Oasis sucks! I always wondered why so many people like to level the teens there- the money is mediocre at best, and it's full of randomly wandering (and trained) high level mobs. Seems if you want a group between 12 and 20 though, you will probably have to go there.