#3 - Castiel
Castiel is arguably my favorite Supernatural character. I say arguably because sometimes my favorite character is Dean. Most of the time trying to choose between the two is like Sophie's choice.
I first saw "Lazarus Rising" which is the episode that introduced Cas a couple of years ago. I can still remember the shock and awe I felt at his introduction. His entrance onto the show was one of the best of the series. They built it up with a lot of mystery and anticipation and it really delivered. I ended up spending the next hour or so after looking up Castiel on Google to see if he was a real angel in myths or not. (He is the Angel of Thursday in some lore which is amusing since Supernatural was airing on Thursday at the time.) I was so curious because, to be honest, I was right there with Dean. I didn't believe there were angels in the Supernatural universe either, but I'm glad there are because I think their introduction brought new life to the show.
The thing I like best about Cas is the journey he takes as a character. He is often an antagonist to Dean, but never truly against Dean. His character has gone from blind obedience, to doubts, to rebellion, to being lost, to a lack of faith, to misplaced faith, to a prideful assumption of power, and back to lost. Every step of that journey has made complete sense to me. Dean often calls Cas a child and in many ways he is, because for the first time in his existence he is having to learn how to stand on his own without a higher authority to command him. He was bound to stumble and fall, and when you are as powerful as he is those stumbles have huge consequences. But the thing about Cas is that he is at his core a good person...at least as good as anyone on Supernatural is (I love the shades of grey on this show).
I also love what Cas brings to the show in terms of humor. Supernatural has always been a dark, angsty show with humor to lighten it and I love the way they play Cas's fatalistic cynicism about everything for laughs. He also brings humor to every day things. He doesn't understand human social convention so things as simple as personal space or using a telephone become hilarious. Misha Collins plays the role perfectly highlighting these differences between angel and man when he played Castiel's vessel Jimmy Novak.
Supernatural doesn't have a lot of secondary characters, but the good ones always challenge either Sam or Dean in ways that enrich the show and Castiel does this particularly well with Dean. Castiel's relationship with Dean is one of the highlights of the entire series for me. The fact that he effectively replaces his faith in god with a faith in Dean is powerful, yet doomed to failure because Dean is a flawed man. The characters play off each other well, bringing forth different aspects of Dean's personality. Cas gets less screen time with Sam, but almost every one of their interactions either sticks out in my mind for its humor or angst...or both.
Basically, I adore everything about Castiel, good and bad, and the things his character brings to Supernatural. I hope that they will continue to allow him to grow and develop in season eight, even as I prepare myself for the worst because he is a secondary character in Supernatural. Things rarely work out well for them.