Halo 1 2 3 and ODST. Even after a long break, they're really really in my muscle memory. Unfortunately, none of the games offer me any challenge whatsoever on Legendary anymore (nope, not even Halo 2, so goddamn easy, i remember all the snipers and more difficult enemies etc.).
Mass Effect 2, i played it so many times.
Mass Effect 1's shitty gameplay and unstable framerate on 360 made it somewhat more difficult but fortunately the game's poor enough a game it can be completed on hardest difficulty without upgrading gear without any real problems (just takes longer).
Starcraft/Warcraft 3/Command&Conquer 1/Red Alert: Mass over-large army, obliterate enemy.