Well it's fun to reminiscent, so here is my list. It's ruled by characters who are the quirkiest, either in their personality, mannerism or something in their background. In no particular order:
Seanna on Shadowed Isles - the Fianna Galliard who was the scrappy tomboy of the pack, and gradually matured into her responsibilities to pack and sept (even respected!). I eventually retired her along with the rest of her pack, but she had arguably my most complete PC life-cycle and immersive play with a clearcut start and finish to her story. She was in a few adventure scenes that were BAR NONE the most memorable I have had with any Mush PC's (i.e. the Tibetan scenes, for instance), so that definitely helps cement her as one of my favorite characters.
Interestingly, Seanna is also the only character on this list that had a romantic relationship as part of her regular RP. Wherever you are, Gideon, hi!
Marianne Beaufort on Denver - I didn't play Marianne very long, but gosh she was fun. She was a Ventrue from London, and showed up with her coterie consisting of her various childer -- she was the elder of the group and brains of the operation, but she posed as the youngest of the bunch so people would underestimate her. The main fun I had with Marianne was her extensive and ridiculous vocabulary: she was infamous for using big words, archaic words, obscure words, and occasionally fabricated words to confuse everyone. I had to do a lot of research just to have a list handy for it.
"They want to interfate my gaudiloquence? I say pish-posh to those oncethmus aretalogers."
Scissors on Generation-X - Alas the game was short-lived, but Scissors was a 15 year-old Bonegnawer Ahroun who had no money, a drug-addict mom, and Rage-8. She was bubbling with anger all the time and physically shaking with barely constrained Rage; the fun part was her constant struggle against just blowing her top and killing someone outright, because she was the responsible one in the family. I liked her so much Scissors has been my namesake in Mushland.
Sydney Rae on Darkwater - Sydney was a mortal on a Changeling game, but also a Federal agent who was looking into weird stuff. Her story was a tragic one - her toddler son was taken by the Gentry and eventually someone app'd him as a PC (as a grownup, of course), unfortunately soon after I lost interest in Mushing and faded away. She never really found out about Changelings, though she knew many of them and figured out some of them were paranormal in some ways, but never dug too deep because she befriended some and they helped reunite her with her lost son. In my years of Mushing, Sydney was the one character who I could see as a viable TV/movie lead.
Mad Jack on Oathcircle - A barely literate Redcap with Intelligence score of 1, Mad Jack was more an attack dog than human. He was crazy, violent and disgusting, but also childlike in demeanor and I played him as comic relief. He seemed well-received by the Freehold despite all his negative attributes, and he had some very memorable and funny scenes. Definitely lots of fun to play.
Marie on The Reach - In some ways Marie was the most grounded, most level-headed characters I've played. She was a Psychic/Wolfblood who was, at best, on the periphery of the werewolf sphere; the lion's share of her RP was with the Law sphere, and she had a lot of memorable adventures as a beat cop. Even as a police officer she was mainly professional and taciturn, and eventually her psychic abilities and gunplay skills grew to a level that she could beat down most run-of-the-mill Supers, but it was mainly the police RP that made her fun for me.
I have been looking for him for a year now!
And Elgana was the best.