You know, one of the biggest differences that I have found in regards to 3e vs. AD&D - in AD&D I preferred running games for levels 1-5 the most. (Sometime after the wizard got his second fireball it seemed like the players' minds were turned off...)
In 3rd I have run games all the way to 15th level, and enjoyed all of them. The characters seem a lot more varied, with none of the cookie cutter approach that I found in AD&D. If I had to piclk it would be 3-12, but I have liked them all.
The Auld Grump