Sackboy's level 3 appears to be the best IMO. He's faster to get kills. But I seriously don't get why it was OP in the first place and why they nerfed him :/
Dante's and Nariko's are very good, they can get a lot of kills quickly. Probably to compensate for the other supers lol
Zeus' is pretty good as well. Although easy to predict.
Hmmm.... this is a tough one. I've seen a couple of Sweet Tooth players who were able to get 9 kills with their level 3 (this was before the patch BTW). However, I've also seen a couple who were only able to kill one single person. Same thing with Kratos. Sometimes, they get up to 6 kills, and sometimes, they only manage to get one. It all comes down to the skill level of the player basically, so I can't really decide what the best level 3 is at the moment.
As for worst level 3 though, I'll probably go with the screen clearers (Heihachi, Parappa, Toro, Spike).