[The Fifth Horseman]

That organization is Mutant Alliance, if I recall.

Personally, I don't find mutants too good soldiers.

S.E.L.F. or Sentient Engine Liberation Front on the other hand can supply you with androids. In the first stages of the game, they are incredibly powerful - far better then ordinary humans, in face. Increased strength, vastly improved accuracy and complete immunity from psionics make them quite the soldiers to send in when you have an alien psionic to deal with, too.