ill have to say a tie between:
Super Mario Galaxy 2
Monster Hunter Tri
MP: Corruption

but that is from what i have played. i plan on getting RS2 and RE:darkside chronicles soon though.

Mario Galaxy 2
a boy in his blob

Super Mario Galaxy 2

IMO Metroid Prime 3 sets the current graphical bar on the Wii although MH3 isn't too far behind. I don't know if Other M will beat it.

Okami

3D: Monster Hunter Tri

2D: Muramasa: The Demon Blade

Uh what? o_O

As for the OP I wouldn't really put one above another there are a lot of games that look good but to be honest are miles behind what they could do on Wii, Muramasa is an example of this, so depending on how you want to look at things it goes like this.

Technical:

Super Mario Galaxy

The Conduit (yes yes bland art and design doesn't mean the engine didn't do some crazy things that even SMG didn't do)

Call of Duty MWR

Overlord Dark Legend

Art:

Monster Hunter 3

Muramasa

Okami

Super Mario Galaxy

Note that most of the games in the artistic section look better than the technically impressive games simply because of art and design, but games have always been that way.  If you look at something like Red Steel 2, many of the bushes and enviornmental objects are really basic but its using the style to cover that up.  Monster Hunter 3 has circle shadows and the lighting isn't real time, there are no shadows cast, etc but the openness of the levels, the textures, and models, it really captures the sense of nature, probably one of the best games this gen in that respect, up there with Battlefield 1943 and Crysis (not graphics please don't get that wrong I'm talking about how it reminds you of nature, I'm a whore when it comes to nice nature scenery and all those games depict nature scenery in an almost artistic fashion like a painting you'd see)

You forgot that it looks better than RE5 too haha RE4 just has a lot of style and looks really good, I'd put it up there with some of Wii's best, there is more juice to be squeezed out of the Wii than what RE4 does but RE4 is a lot like SMG a good mix of both artistic design and technically impressive visuals unlike its successor which lacks on the artistic design and relies more on a engine that makes the game look to sterile.

Muramasa hands down....too bad it doesn't sell well...really nice game