I've not worked it out, but I suspect that the Training Hall (Thjorgard) takes some beating. This particularly true if you're playing the game without the v1.3 patch, because the place respawns every time you enter. With the patch, it respawns every month, which is still pretty good. Of course, if you want to, you can force a respawn of any area by leaving it, deleting the minisaves applying to it, and then re-entering it. I personally wouldn't, but if you want to play that way, you're perfectly entitled to do so.
Also, if you have a Wizard or a Lich, it pays to get them up GM in Learning as fast as you can. It won't help the rest of the party, but it will do wonders for them.
HTH! 
