You'd have to tell us what types of games you are into, but innevitably it's a matter of personal prefference. And no matter what game you choose, you'll always find people declaring it the greatest game in the history of games, while others call the same game the worst game they've ever played. But, if you're interested in my pics:

Star Wars: Knights Of The Old Republic (1 and 2, but definately play 1 first)

Star Wars: Battlefront 2

Splinter Cell: Chaos Theory (and soon to be released "Double Agent")

Halo 1 has a beautiful single player, but Halo 2's is awful IMO (much too linear, very closed in environments, the vehicles and physics are ruined, you can't "feel" the weapons like in Halo1, overall the whole experience is charge in and blast away, rather than figuring out different ways to approach a vast-environment situation).

Jade Empire (absolutely beautiful environments, and tonnes of ancient chinese philosophy, culture, and mythology).