Favorite Legionnaire, eh?
My personal favorites were the characters that Jim Shooter created: Karate Kid and Princess Projectra.
Karate Kid was one of the first DC heroes to have a personality more complicated than a "serve and protect" Silver Age mentality. He was hotheaded and loved to scrap. He also had a wonderful backstory around him, which involved the wonderful Curt Swan drawing a futuristic version of Tokyo (Curt Swan should have drawn a Samurai comic; his art in that one particular comic was so stylized and special). Jim Shooter characterized him with a very light touch: he was chivalrous ("hitting a woman! If it wasn't for the Legion code, I swear, I'd show you some REAL Karate!") and easygoing.
Kid and Projectra had the most wonderful of the Legion romances; it was slow coming, and amazingly, showed what a complicated character Karate Kid was; he wasn't just a scrapper, he was exceptionally sensitive ("you remind me of someone I knew a long time ago, you brave little princess.") and he gave way to painting and flower arranging.
Princess Projectra had an incredible degree of sweetness that instantly wins one over. She was able to convince Validus to join the superheroes because of her showing that weird monster kindness. She was proud and gutsy in the face of danger. She also was mildly concerned with what people thought of her: notice, when taking the Legionnaires back to her homeworld, she was slightly embarassed of how backward it was, and told her family the Legionnaires were all "nobles" from a distant world.
Projectra also had the most amazing power, at least visually: nobody could draw creatures, hallucinogenic vistas and monsters the way Curt Swan could; anybody that calls Swan's art "boring" ought to see the panel where she surrounds the Persuader with quicksand, a bizarre alien landscape, and a giant snake. It makes one sorry that Curt Swan never did a mystic comic; he would have taught Ditko a thing or two!
God, I love Matter-Eater Lad. His power was so demented, how could you NOT love that guy?
I also loved Quislet; he was gossipy and egocentric, and had a truly bizarre power. The fact that we never actually SAW what Quislet looked like is very interesting.