Most interesting companions would be Carth, Jolee and Mission because of their story and dialogues, most useful companions would be any two jedi for combat prowess, most fun companions would be Hk-47 (when restoring memories) and Bastila (at least if you were male and flirting with her)
Canderous actually turned out to be a killer melee fighter, dual-wielding some nasty blades. A shame that he turned all soft in the end.
Least interesting, the walking carpet and the mobile drinks tray
Just my experience from playing 3 times through (1 nice male scout, 1 bad male scoundrel and 1 nice female soldier to check out alternate dialogues). Will probably play it through once more as bad guy, as it was more challenging.
Edit: Oops, forgot Juhani, never figured that one out. Was she actually hitting on my female character ??? :ph34r: