Very tough question, there is no single best. There's a lot more to 'good graphics' than making a multi-million polygon model then slapping a super high res texture on it.
Style, detail, colors and special effects are all very important. Character and environment animations are also very important; jerky animation would ruin a game visually even if it had infinite polys with infinite res textures.
Most of the good ones were already mentioned above. There's also some genre-based 'restrictions', for example, massive scale war type games (many RTS) generally have low polys on units so it can run better on most systems.
MMO's also generally sacrifice polygons/textures and animation to run better on a wider variety of hardware. Only one has impressed me with some of its visuals and that would be Aion; by far the best character detail and animations of any MMO. Kind of got me spoiled and now I can't stand to look at the terribly repetitive, crappy combat animations of other games in this genre.
Best special effects generally go to Squaresoft (now Square-Enix) titles, they have many talented artists that seem to always stay ahead of each generation.
Graphically best RPG overall would probably be FF13 if it was ever released on PC by some rare chance; it's decent on ps3/x360, but could be way better if they give it a proper release on all platforms.