Okay well sorry to hurt the fanboys but the Halo Trilogy is badbadbad.
Team Fortress 2 on the 360 is /puke compared to on the PC.
And Fallout 3 is undoubtedly one of the best games that has come out recently, you will play for hours on end, especially with all the free downloadable content (don't know if the DLC is free on 360 but on my PC is is =3) and you will have a blast leveling your character and deciding between good or evil, friend or foe, or kill em both and be neutral! Limitless possibillities for your character.
Get Fallout 3.