Super Mario Galaxy has the perfect Mario, hands down. The Smash series may add tons of detail to the textures, but none of that is needed, and it actually detracts from Mario's simple charm. NSMB is probably the reason why Galaxy got so many of the character and enemy models in the game right since it took what was old and made it new again, so I have to give credit to that game too.
When it comes to sprites I must say the original Super Mario Bros is the best. Simply because of how iconic they are. Pick a frame, any frame, and you'd be looking at a classic image. SMB3 and SMW are great too, but they don't have the same impact as the side view head and frontward body position of the original.
Mario and Luigi is legendary in the spriting community. It started a huge style and is still highly regarded. The sequels however, although great, put Mario and Luigi in these strange positions with odd looking walking animations... Could be the babies faults, but even in the third game its still present. So yeah, the M&L games have great looking sprites they still aren't what I would call an ideal Mario. They fit the style of the games great though.
Mario 64 and Mario Sunshine... I just don't know, something about them isn't right to me. 64 of course was very limited since it was the first fully 3D Mario, But still, there were some strange things to him. His sections were too separate and apparent. Sunshine was just a different looking Mario altogether, so I can't really say I'd prefer him.
The spin-off games are passable, but a little bland. As technology is getting better so are the character models. But they just got it right in Mario Galaxy... Man, they got everything right in Mario Galaxy. I don't mean to overstate it, but they really just did get it all right.