Could have sworn there was a thread about this, but I can't find it either.
I assume you mean
other than Usagi, right?
I have to divide my favorites into two categories: favorite supporting characters, and favorite characters that work on their own.
Favorite supporting characters:
Jotoro -- he brings out so much about Usagi. His mere innocent presence and anxious desire to follow Usagi's lifestyle makes Usagi question the life he has chosen and the pilgrimage he is on. He also makes even the most pathetic of desperate bandits seem more menacing when holding a knife to Jotoro's neck. Simply put, no other character adds more to Usagi's personality and world. I only hope that, as Jotoro grows and learns, he will continue to be a compelling character in Usagi's world. Maybe Robin or Nightwing to Usagi's Batman?
Gen -- He plays off of Usagi so darn well as the antithesis of everything Usagi is, and he's a lot more complex, thoughtful, and serious than he appears. On so many levels, I think Gen is actually a deeper and more reflective character than Usagi is, and the day that Usagi finally realizes that will be a powerful one. Gen's the kind of character who's presence fundamentally changes who Usagi is nearly every time they meet without Usagi even realizing it.
Favorite characters that work on their own
Katsuichi -- He just kicks butt on every level.
Sasuke -- What's not incredible about a demon sorcerer fox? He's powerful, has a very intense and enigmatic persona, fights fascinating spirits and magical enemies, and always leaves more questions about who/what he is than he answers.