Mass Effect without a doubt in my mind looks the best. Especially how well animated their faces were during conversations. Assassin's Creeds get the nod for close second. Fallout 3 in my opinion doesn't even come close to looking the best. Oblivion frankly has better looking environments in terms of textures.

Mass Effect, (not so much now, but that's only because I've played through it like 5 times already) Oblivion, Fallout 3, and the King of taking up my time...Morrowind. I haven't spent more hours on one game than in Morrowind. Greatest game I've probably ever played.