Tough choice but as far as I'm concerned my number one all-time favourite would be Giratina :D I think it's 6 legs are kinda cute too in a way :3
Back in Platinum, catching Giratina in sky forme, was really something else and a great experience to travel to the Distortion World :D I think he knows some awesome moves too, and even in Super Pokemon Rumble he is a great Pokemon to Battle with
You might want to check what forum you're posting in ;)
Not just from this generation, but Rayquaza is probably my favorite legendary Pokemon of all time. Everything from its residence in the Sky Pillar, to the fact that it has the ability to fly around Earth for thousands of years at a time without having to descend were pretty cool to me as a kid. That, and the battle music for it (along with Groudon and Kyogre) is still my favorite track in a Pokemon game, as it really captures the intensity of the moment.