Even the ruthlessness of SkullGreyMon wouldnt compare to Wargreymon. Completly different levels that Skullgreymon couldnt dream of touching (Until digivolution). Anyway, this fight is kinda up in the air. I think of pure destruction and fighting potential, Wargreymon has this. But if Mewtwo is able to mind control Wargreymon, I think Mewtwo would win.
Though on a biased opinion, I would want Wargreymon to win. Miss Digimon. Very good show.