MGS is at the top for me. Probably my favorite part of all the games is going for the 0 Kill/0 Alert completions. I also like how 3 and 4 have sooo much you dig around and find to expand your gameplay options, I probably still haven't seen everything there is in those games.

Played Tenchu when I was really young. Don't remember too much, but there really weren't a lot of games like it.

I like the stealth sequences in the Modern Warfare games, although you're really just following someone else.

Stealth in Oblivion was really fun.

I played through a good amount of Splinter Cell conviction on PC, it's a pretty tight game. The co-op can be fun as hell, I really wish they'd make a console Metal Gear like that (Ground Zeros maybe?). Peacewalker was fun, but I really want a big HD one like that.