Rainbow six series are dead but I find Far Cry 2 (developed by Ubisoft too) very good because stealth approach works, and Far Cry 3 released few days ago on November 30, 2012. I'm so going to play it after my final exam finish.
It's different from Call of Duty because in COD, no matter how far and stealthy you are, the enemy still sees you. But in Far Cry 2/3 if you wait for the sun goes down and choose silenced weapon, you can take down enemy 1 by 1 without getting any attention, the way you use stealth attack to make enemy panic makes it so different from COD and makes this game so much fun.