While the Rock and Cena are obviously ridiculously strong, don't sleep on Seth.
Shermie 97, Paul, Lady Chang, Schoolgirl Ling
They give the player an attack boost by a whopping 60%, the only caveat is that with Shermie/Ling you must remain at over 65-70% health, and with Paul there is a health penalty of -10%. Invaluable for those boss missions! However, with Shermie, it becomes harder as you will need an almost perfect run. The other problem is when you need three strong or element specific characters for a mission... Still, using either of these two leaders, Inferno Chizuru can be done with the two remaining.
Kyo 95, K' 99, P.Yashiro, Haohmaru, W.Iori, S.K', Kazuya, H.Heidern, May Lee
These characters will give a 50=55% attack bonus to characters who share the same element (or series) and health is not an issue.
She gives your characters a 20% attack boost and a 25% boost to the ultimate attack. Does it make a real difference compared to the other leaders, say when attacking within the same timeframe? With criticals in the mix, it's hard to say.
50% extra burn damage
Bleed 40% which makes the EQ.Chizuru boss a walk in the park
Kim 98, Andy 96, Robert 96, Chin 96, Mai 94, Yuri 94
Critical Damage 40-65%. Kim gives the bonus to all, the others are element specific. This is useful for the Mecha Goenitz boss
Don't forget you can look through the whole list from Shifty Hustle's spreadsheet