I would have to say I absolutely loved the original Splinter Cell, soo much patience and effort goes into that game. Pandora's Tomorrow was also excellent.
Nearly all of the Tom Clancy games I have played are great. I love the first three Rainbow Six games. They are just absolutely awesome. Vegas and Vegas 2 aren't that good but I guess they are all right. I love it how playing To Clancy's games take a little patience and planning.
Definitely the Splinter Cell games. The old ones are wonderful, and the new one on the 360 is great too. At first I thought it was going to be bad because most new games being made from old series have been like that to me for a while, but when I played it I was amazed.