Metacritic averages the scores on Xbox360 games from many review sites. See:
http://www.metacritic.com/games/xbox360/
It obviously depends on what genres you like. Bioshock is a great FPS with role-playing elements (the demo on Xbox live gives a good indication of gameplay). I've been playing through Mass Effect over the last few weeks and that is very good fun if you like a good story / sci-fi RPG. If you are into driving games then there is always Forza 2 and Colin McRae DIRT. Call of Duty 4's single player is short but extremely entertaining and memorable. If you have 60+ hours to burn and you like RPGs then I agree that Oblivion is a must.
Just my 2c