Halo: It is the beginning of the series and it is very fun but yet simple it is for XBOX.
Halo 2: It becomes harder but now your also on the covanent side this time as well as on the good side. Halo 2 has harder enemies more vehicles and alot more fun than Halo. It is for XBOX.
Halo 3: It is the best halo game yet it is very awesome there is more vehicles
and the enemies are harder it is still very fun This game is for XBOX 360.
There is also a rumor of Halo 4 coming out for Xbox 360.