I've played Rainbow Six and Rainbow Six 3 as well as Splinter Cell 1-3. They are all great fun. I really like Rainbow Six's mission planner, thouth things rarely go the way you want... it's awesome when everything just falls into place. SC is a lot of fun too... recently it's become more action than stealth (though you can still play it stealth-like if you wish), but it's still a lot of fun. The online mode is very interesting as well. SC3's co-op mode is a lot of fun too!

Ghost Recon: Advanced Warfighter just looks HOT! I can't wait for the PC version to come out. My friend has it on 360 and says it is awesome (but buggy).

I really like Tom Clancy game because I'm into all that military stuff... and his stuff is based on current or upcoming technology, so it has a kind of authentic feel to it... like it could actually happen.