as far as i know, there are 2 UCs considered to be the best. One of them is Dr. Saiphon and the other one is Beth. Beth is probably UC with best stats, which you can effectively farm in a storyline.
You arent that far from having the chance of recruiting her. You would have to beat SS Gloria and 1 more to get to Water temple where you can recruit Beth.
It has been said quite few times already, but depends on what you prefer to play. PvP difficutly is based on your level so going further in the story and thus leveling a lot might hurt you. If you are going for Epic Boss just go for Beth i guess. Altho you would be fine just going for any UC and max-fusing it using any given formula. I just managed to beat Epic Boss final form with 2 maxed out UCs (Saiphon and Beth), 1 semi maxed UC (700 atk) and 1 semi maxed rare (650 ATK).