The most fun I've had in a stealth game was definately the original Tenchu. Splinter Cell and Thief were alot of fun, but Tenchu was the only time I ever really felt lethal lurking in the shadows.
I like the Splinter Cell series. I guess cause I can move the camera freely. That's one thing that ticked me off about MGS2. I did enjoy MGS2 alot, had a pretty good story, but at times it seemed more about the story than about the game. One thing I do like about Splinter cell is the ability in darkness for the enemy to walk right by you. That has always provided some intense play. the only thing between you and your enemy is the lack of light and your five-seven.