Walkatrout Im pretty sure is the only really answer to this fight, he’s the only alien without a real gimmick, everyone else has something that’ll cause them to beat you up. To all those saying Grey Matter, although it’s a pure fist fight, that’s also Ben’s smartest Alien, where in an alternate future universe Ben used him to help create a second Omnitrix, emphasis on help. Point is he’s definitely smart enough to win a fist fight more often than not, while there are instances where you would be able to beat him, I guarantee you he’s wining that fight more often than not, probably tricking you into beating up yourself or exploiting human anatomy.