If you could only play five games for the rest of your life-

[Jay520]

Left 4 Dead 2 - Some of the best multiplayer I've played.
Mario Party - Extremely fun in groups
Smash Bros Brawl - Same as above
Starhawk - It's not released yet, but based on the beta, it's worthy to be on this list.

Can't really think of another game that I could play forever