Stealthy archer/rogue in Elder Scrolls: Oblivion. It made the game much slower, but it was very satisfying how you could shoot a hanging log, making it swing back and forth and knock out enemies. And just hiding in the shadows and sniping was a fun feature.
Human Noble Rogue in Dragon Age Origins was fun too, though that was mainly because of the story. Dragon Age Origins has a pretty dull stealth system.
Wizard is a great class in Dungeons and Dragons Online. Unlike most MMOs, the classes feel very unique. The wizard got maybe 50 spells, and could do all sorts of interesting stuff.
I played City of Heroes for a while, and I was an illusion controller. Very fun. I could sneak around invisible, and then pop out at the boss, summoning a bunch of illusory creatures to fight with me. And super-jumping was great. Are you starting to get the idea that I like stealth in games?
I enjoyed playing a team of four blackbelts in Final Fantasy 1. Very hard to get started, but ridiculously powerful after a while. I think I played through that game with four of each class, except white and black wizards (which I did 2 and 2 -- black wizards can't survive without healing).