I've actually been compiling a list of all the Legend Rank spirits in terms of power. There are 85 Legend Rank Spirits and (as far as I can tell) two Ace Rank Spirits of comparable power (Maria Renard and Hero Chao)
Big Boss was my strongest Spirit for quite a while, and in terms of Team Power, the 5th Strongest overall.
The top 5 spirits are Galeem, Dharkon, Galacta Knight, Akuma and Big Boss, in that order.
However, Galeem, Dharkon, Galacta Knight and Akuma don't have Skill Slots, meaning Big Boss ends up stronger in terms of Team Power when given Support Spirits (Galeem and Dharkon have 13,640 Power, but Big Boss ends up with 15,399 Power with two Legend Rank Support Spirits)
The strongest Grab Type Spirits (in terms of Team Power) are Mega Diancie and Isaac (Dark Dawn), who have several characters above them in terms of base power, but because they have Three Slots, they can technically get stronger than them. By this logic, the strongest Spirit is Soma Cruz, a Neutral Type who has three slots, and he's the 16th Strongest in terms of base Team Power (Lyn is #15).
Cutie J is the second strongest Attack type, and Zelda (Ocarina of Time) is the second strongest Shield Type, both have Two Slots, meaning they're technically more powerful than Akuma and Galacta Knight.