Call of Duty 4, a great rental. The single player game is really short, though it does have some replay value in online multiplayer. If you've got Halo 3 though, probably not worth buying.
I second Hoogs recommendation for Crackdown, though I think it's worth more than a rental. It's probably one of my favorite games on the console. Especially once you get past all the really big names.
Top 5 in no particular order...
1. Crackdown
2. Viva Pinata
3. Beautiful Katamari
4. Gears of War
5. Guitar Hero
And if you're looking for a great game to download... I strongly recommend nabbing Psychonauts. It's a XBox Originals game. It's made by Tim Schafer. He's one of the guys behind Monkey Island and Grim Fandango, two of the best PC games ever made if not two of the best games ever. Also some great XBox Live Arcade games are Puzzle Quest and I really like the new PacMan CE.