Half-Life 2 has aged like fine wine; I find it difficult to believe how 80% of games produced today still can't match the FPS, photorealism, and aesthetic of HL2, even though it was released in 200...4?
In any case, my personal nominations "most immersive graphics" (not necessarily BEST) go to Assassin's Creed 2 or Gran Turismo 5. The atmosphere of AC2 was almost tangible, and it seriously made me feel as if I was roaming renaissance Italy. Gran Turismo 5 on a hi-def tv... damn. It's a fine piece of work. (not that it makes it an incredible game, but I practically drooled when I first played it.)