When talking about the best Street Fighter 4 moves, and which Ultra combo is the best, there are a few factors that you need to look at. 'How easy is it to connect with and actually hit your opponent with?' and 'How much damage does it do?' are the main questions that you should ask. Beyond that you might ask 'how good does it look?' and 'does it suit my style of play?'
If you're basing it on damage alone, then there's only one person that you can realistically look at, and that's Zangief. His myriad of suplexes and a final powerbomb can take around 5/8ths of a life bar away! The fact that it's a close-range grab move is even more devastating, as it means your opponent can't block it.
Akuma's move is deadly and one of the most frightening, despite being very easy to stop and counter. A simple jab when Akuma is 'floating' towards you is all it takes to stop this move, but make sure you time it right or you might not be around for much longer! This nervousness can often be seen when playing against Akuma, and players panic when he does his Ultra move, and end up getting caught in it.
Gouken's ultra combo is one of my personal favourites, if not for the reason that it looks so badass! It's basically a giant Dragon Punch, which does around 10 hits to your opponent. The best Street Fighter 4 moves the ones that can usually be activated easily at the end of a combo. For example, because Chun Li's Ultra combo comes out so quickly and connects very early, you can time it so that there's almost no lag time between your build-up attack and activating the combo.