Ok..... here's my list.
(I subscribe to the XBOX magazine, as it comes with demo disc's, ~and check out every game before I purchase it.) Anything jumpy or with poor graphics does not even get a look in. Playability (or ease thereof)is the clincher. Pays to checkout what users have said in game forums too, but ultimately the decision comes down to whether you personally like the game.
The List
(My top 5)
Tiger Woods 2003
Tiger Woods 2004
V8 Supercars
Halo
Splinter Cell
(The rest)
Red Faction
Mafia
Moto GP
Hitman 2 -silent assassin
Colin McRae Rally 1,2 and 3
F1 2002 (the best release out of all this series, imo)
Ghost Recon
Morrowind, The elders scrolls (Massive game... just massive)
Outlaw Golf
Myst III ~Exile
Medal of Honor ~Frontline
The Sims
Rallysport Challenge
(and a couple not worth mentioning... too old)
Some of these games get traded back in when I'm done with them. I have a great deal with my local supplier, and get around half the cost back again when I trade up to the new one. Trick is not to own the game too long before you trade, and it has a great chance of resale.