Depends which system (I, sadly, have way too many):
Sony PSP: PSP movie Super Troopers, Final Fantasy 6 on the memstick (I love homebrew) - Super Troopers is great if you love mindless screwball comedy, and of course Final Fantasy 6 is just classic.
Nintendo DS: Advance Wars Dual Strike - An astounding game that makes great use of the Dual Screen, the turn based nature of the game plays out like a well played game of chess - But with Tanks!!! Super fun and super addictive.
Microsoft X-Box: Crimson Skies: High road to Revenge - A pretty good flier with lots of replay and cool planes. Plus I needed a break from constant Halo 2.
Sony Playstation 2: The best game of all: Katamari Damacy. The premise: Your father, the King of the Cosmos gets drunk and goes on a bender, and while doing so inadvertantly destroys all the stars in the sky. So he sends you( who are this tiny little guy to Earth with a round ball callled a Katamari. I don't know how the ball works, but when it rols over small objects they stick to it. The more objects it picks up, the larger it gets. The larger it gets, the bigger the things it can pick up. The goal is to make your ball of objects a certain size within a certain amount of time per level. It is extremely addicitive and great turnabout when you discover you are large enough to pick up that pesky elephant that has been batting you around half the game . In the last level you start out able to pick up flower pots and end being able to rip whole islands and continants out of the ground. Fun.
