there is no best 360 game, although I do believe there are selection games you must own to justify having an Xbox 360.
titles like
Bioshock - Its under £10 in most stores, the original masterpiece, dont bother the sequal unless its at bargain price, it feels like they are just forcing more of the same
Gears Of War 1 & 2 = Both of these games are under £15
Halo ODST - With Halo 3 being dissapointing, ODST was a dark beautiful alternate adventure, normally under £15
Mass Effect 1 & 2 = The most immersive story in a game I have ever played, you need both these titles to eat away 100s of hours of your social life, again should cost no less than £25 for both titles
If you like Football titles I would wait untill FIFA 11 and PES 2011, its all about personal preference with those to titles, I have always liked PES, but I loved the EA world cup game this year.
If they use Be a Pro on Fifa 11 like they used Captain your country on world game, then Fifa 11 with have me sold
Left 4 Dead 1 & 2 = Frantic 1st person Zombie action, if you going on Xbox live, these games are a must