on my first toon (smuggler) i hit level 50 with only Corso's quests completed, so i felt closest to him. however, i started raising the affection levels of my other companions afterwards and taking part in their convos. i started out hating Risha, but she grew on me...
so Corso and Risha are my smuggler's favorites. i actually love all the smuggler crew, but if i had to pick, i'd keep the first two over the others.
as far as my jedi knight goes, i haven't finished all the convos for all her companions yet, but Kira is definitely my favorite of them. not only did i level to 50 with her and her only, but as we traveled around, it felt like i was visiting new places with my bff. and i love her sense of humor. Scourge grew on me as well, but not to the same extreme that Risha did. i just have more of a high respect for the sith.
Blizz is my favorite BH companion. i like Torian and Mako too, and Gault kinda grew on me, but Blizz is just so freaking cute. i wanna squee everytime i use the companion dance animation with him out.
i haven't finished all the consular companion convos yet either, but the only one i really like is probably Qyzen, and i think it's mostly because he stuck with me from the beginning. i like his devotion to the scorekeeper and you, and i find the tradoshan customs interesting. the other consular companions are kinda meh to me.