I like the cartoony and colorful art style. Be it pixel art, low poly PSOne graphics or something like Rayman Origins/Legends. Like Aura points out, though, not any jumbled mess of colors would do; the colors should be carefully selected and contribute to the mood of the game or level.
While I find Super Mario Galaxy and it's sequel really beautiful, I'm not
that much a fan of the overly cartoonified and saturated objects, characters and certain landscapes, which - intentionally, I guess - make it all feel like a toy box come to life rather than a stylized, living universe.
My absolute favorite I'd say is Rayman 2/Revolution:
The mood and atmosphere in this game is just amazing, with backgrounds reminiscent of animated Disney classics. Despite the low polygon count, the somewhat quirky shapes and beautifully painted textures have a certain "texture" to them that along with a well-chosen set of color palettes make it all really come to life.
I think there's this fine line of it feeling simply
natural. Not too realistic, because then most everything would need to be realistic to naturally fit in, and not too broken down to basic shapes and primary colors like Mario, because then it rips the immersion away.
Obviously, this preference doesn't strictly apply to every kind of game. Certain games require certain art styles: A quick and fun but shallow arcade game should probably reflect these very attributes with similarly shallow graphics(though not ugly or sterile). A game of Snake, Tetris or Asteroids with hyper-realistic graphics would feel somehow off. GTA would lose it's balance of mature seriousness and immature brutality if it didn't have a certain degree of realism.
Edit: Looks like I missed the point about this being only about 2D art styles. But my point remains... unless this is more about the technical art style, like pixel, vector and high resolution painting or photo realistic. In which case I can't really make up my mind between pixels and hand painted things. They're both able to well convey the semi realistic cartoony style I like, while vector graphics have a tendency to be somewhat lifeless or sterile.