I've had lots of fun in 4th edition as both a wizard and a warlock (eladrin and drow/dark pact, respectively). Playing a predator druid was also fun, but a little frustrating at times in my last campaign (mostly because the DM was giving a lot of help in and out of game to the 2 new players; slightly overpowered builds, environments/encounters that they would be especially good at, etc. That would have been fine, but it was annoying when the land-of-undead-that-only-the-cleric-is-effective-against stretched on for weeks).
In 3.5, I like druids and wizards, although wizards are quite frustrating at 1st level (4 spells per day, only one or two of which does any damage, and 4 hit points). I'd like to try playing a rogue as well.