In X/Y Land, I'm gonna say Zygarde. I like it's crest along the back, and being Green is my color of choice.. still, kind of looks misshapen and kinda sluggish.. I actually don't really like the legendaries (as of yet) for 6th gen all that much.. When Diancie comes out, probably that one.
Outside of X/Y, Rayquaza is amazing in pretty much every way.. wasn't some weird goofy looking blob or anything, just a really kick ass looking dragon. Saddly, I've never had one of my own.. maybe some day.
Lugia's pretty neat, or rather, I kind of like the idea of it. The giant hand wings are a bit silly looking to be honest.
Articuno is also a favorite of mine, its just such a shame that it's so frail there's no real point in using it for anything.