Mr. Fat Cramer is right about one thing, it is more difficult to pick a second favorite.
At one point I had decided that my favorite Legionnaire was definitely Karate Kid, mostly due to his uniqueness. He was the exception to the super power rule. he got in based solely on his training and skill ... no fancy shmancy super powers needed ... AND he trounced an overconfident Superboy ... that counts for alot.
My second choice seems to fluctuate. At various times I've thought of Wildfire, Timber Wolf, Ferro-Lad, and Mon-El, as favorites.
Sometimes you pick a favorite based on a storyline, characterization, or powers. While other times it's about the potential you think the character has.
Most of my favorites are characters that I think have an incredible amount of untapped potential.
Steve Lightle