Personally I would rather have Wildmutt because it gives me a lot of nostalgia. I understand if RIpjaws or Ghostfreak weren't in because Ghostfreaks entire arc in the original continuity. Ripjaws design might be a little too much for kids but I think Wildmutt is fine. I don't know why, I just personally really dislike Overflow.
overflow is a powerfull alien, but ripjaws is cool, and so is ghostfreak, and if ghostfreak is in it he whoud be in all 5 shows. Wildmutt IS a nostalgaic alien, but do you REALLY want to swap overflow for him? remeber, ben can only have 10 aliens at a time in the Reboot, but if you want to swap someone, swap rath ot slapback. big chill whoud be a nice addtion to the reboot, but based on your opptions i'll go with ghostfreak.