I think Ben 10 Ultimate alien most weird alien is Ultimate Spidermonkey. The other alien also has strange look, but Ultimate Spidermonkey is combination between giant gorilla and spider. Original Spidermonkey has power Sticks to Walls and Shoots Webs. Ultimate Spidermonkey has power Super Strength, Sticks to Walls, Shoots Webs. When he go to Ultimate his body will be come big gorilla.

Ultimate Spider Monkey Fan Transformation