The worst possible mutant IMHO is anything that:
-has 0 attack
-is immaterial - can't be remutated
-is not
or
(A nightfall won't help)
Even a mutant like this has one use: it provides healing for empathic bond.
Though in terms of good mutants... the best I got was a crimson dragon with deja vu, the copy had deja vu, the third one had dive. So after everything, I got around 60+ damage per turn