They're all considered pseudo-legendaries or pre-legendaries as some say. They're all right next to the real legendaries(except garchomp but he's counted anyway by stats) and some are even after some in the pokedex like dragonite after the legendary birds and tyranitar after the legendary beasts.
From a purely objective standpoint, Garchomp. He pretty much broke the metagame. The others are more or less jockeying for one another, though Salamence probably edges out on top.
Personally, I like Dragonite the best. Not only is he cute, he also has some supportive functions that can take people by surprise. --- http://z9.invisionfree.com/FEABL/index.php? - Fire Emblem Fans, set your gaze on yonder link!