Gohan oneshots because he is the highest.
I'd say Boo wins. Considering that Piccolo only seemed to make note of Boo's body becoming more appropriate for battle rather than powering up when Evil Boo was formed yet still questioned SSJ Gotenks' chances, I'd say it's fair to say Fat Boo powered up just prior to the split and as such, Pure Evil Boo would be a lot stronger than somewhere over half of the Fat Boo SS3 Goku fought. That said, I'd place Pure Evil Boo as not much weaker than that version of Fat Boo, so anything short of giving Gohan SS3 isn't going to cut it here.