Best Art: Super Mario Kart. Part nostalgia (even though I never bought the game, I've rented it several times), part giddyness from all the character chaos and the personality of old-fashioned 2D art. It almost looks better than the 3D art of more recent games.
Worst Art: Mario Kart Wii. No background, and the two are just floating there holding Wii Wheels. I just think it's cheap that a character holds a Wii peripheral. Imagine Link holding a Wii Remote to mimic shooting an arrow in Crossbow Training. Or Amaterasu holding the remote like a paintbrush for Okami Wii (with various website logos in plain sight near Ammy's body).
Second-worst would be Super Mario Sunshine, now that I notice the pointless text surrounding Mario.