Man, it's been a while since I popped into this thread.
One thing I discovered playing Ultimate is that the Isaac (Dark Dawn) spirit is one of the most powerful Legend Rank Spirits.
Let me explain...
At Level 99, Isaac has a Team Power of 11,672, which can be increased to 15,535 with three Legend Rank Support Spirits (the biggest numerical boost you can give; two and three slot Supporters, and those of a lower rank, give less Team Power).
Naturally, there are other nominally more powerful Spirits, but many of them have two or fewer slots. This limits their maximum power, unless they get boosted further by pairing them with a Fighter from the same series. Some of them, like Akuma, Galeem and Darkhon, are very powerful, but don't have any slots at all, which hinders their max power somewhat (and even then, Galeem and Darkhon have Abilities which are only useful in the Adventure Mode). Akuma has 13.235 Team Power, which is increased to 14,558 when paired with Ryu or Ken.
My most powerful Spirit of the same type as Isaac, Big Boss, has two slots, and has 12,727 Team Power at Level 99.
Big Boss' Team Power increases to 13,999 when paired with Snake without Support, 15,399 with Two Legend-Rank Support Spirits and not paired with Snake and 16,939 when paired with Snake with all Support Slots filled.
As you can see, Isaac is more powerful than Big Boss in some cases; for the record, Isaac's Team Power would be 12839-17088 if he could be paired with himself (or Matthew, I guess).
Out of the Legend-Rank Spirits that I have gotten to Level 99, there's only one character with more potential, and that's Soma Cruz, an Enhanced form of Dracula (obtained by beating Dracula in Adventure Mode, which appears to be the only way you can get him, one per Adventure Mode playthrough); Soma Cruz reaches 17,728 Team Power under the right conditions (Level 99, all slots filled with Legend Rank Spirits and paired with Simon or Richter).
Give it up for our Golden Boy being super strong (and having an Ability which might make him useful against Galeem and Darkhon).