Hmm, really tough fight. #18 is Piccolo's distant rival, considering this latter rivals #17. Gero made a case of constructing #18 more suppressed compared to her brother, so there should be a noticeable difference here. Imperfect Cell was considerably inferior to weighted Kamiccolo, let alone unweighted Kamiccolo. But we have to consider #18 has an infinite stamina and is able to give Piccolo at least a bit of a challenge, whereas Imperfect Cell is a trick fighter, with a good arsenal of techniques. Like I said, it's really a tough decision, but I'm willing to give Piccolo the benefit of the doubt here.