As a kid: Blastoise. Because he makes for an awesome father. And because Water types were my favorite (and still are to a degree) and Blastoise was the most freaking awesome water type.
Close second was Mewtwo. Because it's freaking Mewtwo.
Now: Still Mewtwo. BECAUSE OF REASONS!!! But seriously, Mewtwo just reached a level of coolness that no other legendary has. Because all the other legendaries always just came across to me as uuber-powerful laser blasting behemoths with nothing really special or unique about them. I mean, look at the movies. Lugia is awesome. Latios is awesome. Did they really do much of anything different though? In the movies that is. Latios was fast and Lugia could swim. Other then that...not much. Mewtwo fired off his energy orbs, sure, but he also was a super-jedi, and in the end he just sort of played differently.
Plus, I just really like his design. So sue me.
I've also however grown to like Darkrai, Lucario, Gengar, Kyogre, Metagross, and Braviary. And of course Blastoise will always be one of my all time favorites as well.
There's nothing that sucks worse that that moment in an argument when you realize that you are wrong.