Halo definitely, if you only get one game at first get this - i wont bother explaining, just read pretty much ANY review of the game and you will know why. Its also pretty good on reply value.
Splinter cell, people seem to have forgotten this gem. Should last you a little while - ive played it through 3 times.
Project gotham/rallisport challenge - both great racers. Id probably choose gotham over rallisport simply because there is so much stuff to do and unlock. For multiplayer fun pick ralli tho.
Those are the 3 games i have enjoyed the most, and played the most, and when mates come over halo and rallisport are the only things we play.
Im not sure if its still out but there was a beginners guide to xbox type magazine with loads of reviews and also a demo disc with about 20 or more demos of the best games on it. You might want to pick that up, see what stuff you like.