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.
= = Every day is Kill la Kill day!! = =
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).
Sackboy's is immense but he has the highest meter and doing combos in an FFA or 2v2 match with him isn't wholly viable...especially in FFA. He's spend the whole match trying to get maybe 6 + kills. Assuming he even gets it.
Kratos' level 3 may seem easy to dodge but not if the person knows what they are doing, aside from Sackboy there is practically othing you can do vs a Kratos with level 3 short hopping with the an e blast from the sword. Will net you easily 5 kills. Combine that with the idea that pretty much no one can trade with Kratos coming off the better and you have a formidable opponent.
Nariko's level 3 is also very powerful but her slow AP gain and combos that take a long while to do and that are easily avoidable if one knows the MU make it so that she is in the same category as Sackboy in FFA, 2v2 not as much.
Zeus' level 3 is a guessing game, sometimes it lags so that even if you are in the animation you don't see him move and as such you get hit by a blow you never ever see coming, literally Combine that with the water glitch and Sackboy's teleport glitch and you have an inconsistent super that has the potential to gain 6+kills.
Sweet Tooth is also another behemoth with level 3s. Looking at 7+ kills easily assuming you deploy the robot sensibly so that no one is immediately underneath you when it commences. Combine that with the fact that Sweet Tooth has the smallest AP ratio means he can get that level 3 faster than any other character.
Emmett, similar to Sweet Tooth but a lot less viable because of the fact that he doesn't have a small AP ratio makes Emmett's level 3 very risky to go for. His AP generator does make up for it slightly however
Next is Cole's. Pretty much impossible to dodge save maybe a few cast members and even then it is only for a few seconds. 4 kills minimum on medium to small maps. Cole's long range game and high combo potential make it pretty viable assuming you can pull off the combos quick enough.
This is obviously with regards to a time factor and as such does not refer to stock scenarios.