well, up to lvl 80 there is point visiting mephisto, after that baal runs give the most exp, plus he can drop everything! =)
but I have to admit, I did quite hardcore for couple of years before I found PE.
D2 expansion was/is way better than the old D1. many interesting things implemented and let's say, with magic find over 700 the uber and set items turn quite common - there are very few of the top items left which I still have not looted
J.