I think Mitsurugi's bears mention on utility alone. You can tack it on to pretty much half his moveset, it combos into itself, RO, W!, and scoops enemies off the ground. There are a lot of moves in the game that hit grounded, and a lot that RO/W!. There are not a lot of moves that can RO/W! FROM the ground. It also catches all rolls on wakeup; so you can knock people down near the ring edge, and when they roll to avoid RO...
It might not be the very best in the game (I mean, come on. 8 frames. 8.) But it's easily top three material. See Cervantes/aPat. Damage isn't everything (not that 60 + clean hit is nothing).
I think Nightmare's is worse than most just because of how punishable it is. It's a bigger risk than anyone else takes, for basically the same reward (you aren't hitting anyone with the non-GI version, sorry) plus, I'm pretty sure he's the only one that can't combo into his CE at all whatsoever since he just stands there like an idiot for a thousand or so frames (maybe Damp? lol). Xiba's is much better.
I'm with Ring on how bad the throws are, especially considering the characters that got them. People are already watching for throws against Ivy and Astaroth anyway, and the fact that you can randomly TC under them half the time doesn't help. If you land them they're good, but... so is every garbage unblockable in the game.
Viola's is just sad. You can tell what the developer was trying to do, but I just don't think it's worth it. It should do 40 or so damage and still trap them. Or something.
Tira's just randomly drops at tip range, doesn't it? I don't play against a lot of Tiras in the post patch world (everybody 'round here that played her even a little bit seems to have dropped her...) but I swear I heard somebody complain about it.
Best CEs in my opinion (in no real order)-
Cervantes
aPat
Mitsu
Ely
Xiba