Out of all the classes in all of D&D history, I think my favorite has to be the psion. Psions fulfill basically the same niche as a wizard - they're both smart, intellectual, caster characters, after all. The difference is, where wizards gain arcane power through countless hours of study and poring over ancient, dusty tomes, psions gain their power through using their minds to will feats of imagination into being.
I love the aesthetic of psions, especially how 3.5's Expanded Psionics Handbook art portrayed them: covered with crystals and esoteric tattoos.
As far as powers go, psions have a similar depth of powers to choose from as wizards have spells to choose from. Power manifestation is different than casting though; where wizards must speak the language of magic, make complicated gestures, and precisely handle clumsy material components to cast their spells, a psion has only to think about their powers to manifest them and make those thoughts reality.
Their powers are typically more flexible than a wizard's spells as well - not only do psions manifest spontaneously rather than memorizing powers as a wizard does spells, but augment mechanics make it easy for a psion to alter the strength and/or effects of their powers without having to take all those pesky metamagic feats.
I didn't get to play a psion in 4E, but I absolutely love the psion class as it was presented in 3.5.