My strongest fighter is Rose Knight level 15. So much so that I very rarely use other champs. Where Rose Knight fails, I use Glasswing level 10 or Centurion level 10. My Diamonds level 15 are great for beating champions in the Arena. Valiant hasn't been of much use when I used it.
So, as a sort of advice, I'd recommend leveling a super rare single type champion to level 15 and slowly evolving others.