For me, it's always been Celebi. There have been a lot of mediocre legendaries with Gen IV and V, but none that have made me angry like Celebi did. I never liked how it was introduced as a Mew-knock-off. And an ugly-looking Mew-knock-off to boot. And a Grass-type ugly-looking Mew-knock-off to bring it all home. When I actually got my hands on one, the critter ended up being a massive disappointment in addition to all of the pre-release 'hype'. So... yeah. I really dislike Celebi.