Single-classed only?
Pre-polymorph-nerf-druid
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Cleric
Post-polymorph-nerf Druid
Wizard at very high levels (15+)
I think a case could be made for the Warblade from ToB:Bo9S, simply in that, on paper, he's a fighter-type who's actually competitive with casters at higher levels. I haven't seen one in play yet, though. I'm also not sure if that makes him overpowered or appropriately powered - it depends on whether you think fighter or wizard is the right power level at level 18. That's the only WotC non-core base class I've seen that seems on the high end.
Obviously, some classes are considerably better when combined synergistically...