What the heck!? Celebi isn't rare at all. It's getting to him that's the problem. The question this thread is asking is what is the rarest Pokemon you have recruited, rare Pokemon being ones with low recruit rates. And since Celebi has a 100% recruit rate, it's not rare. The same goes for legendaries like Latios, Latias, Ho-oh, Mewtwo, and Jirachi.
That said, the rarest Pokemon I have recruited is probably Groudon. I would like to recruit a Kecleon, but my team members have a long ways to go before they are Lv. 90.