Hard to pick Luffy's opponent if Zoro actually fight Kizaru decisively. Both are monkey-themed, both are funny, both are acrobatics in their fight.
...hmm how about the "Free Energy Monster" ? Lol i imagine if the "limitless energy source" stuff Vegapunk mentioned is actually something alive and Luffy is the one to beat it, making it submit to Luffy's decision, thus complying to Vegapunk's experiments.