My favourite character that I've ever written was actually Nick in February Stars. That story is old, and it's just a one-shot I wrote randomly one day a long time ago, but I loved writing him in that. It was the first time I'd tried true, first person Nick angst and I had a great time doing it. He's just so sad in that story because everyone else is growing up around him, and he feels so alone, and then when he feels that slight bit of hope at the end... yeah, I just really enjoyed writing Nick in that way, in the bridge between the younger him, and the older him before he got really deep into his issues.
I also love my original character of Brittany in my new story I'm working on. It's the first time I ever did a true character sketch before writing, where I decided her backstory and a whole bunch of stuff. I think it worked out really well because it does make her seem like a more legit character instead of just a random girl who's there to cause drama.