Muffit II, the daggit, was certainly my favourite

I had the exact same reaction to G'Kar. I assumed he'd be some heavy brute who would cause consistent trouble for the crew of Babylon 5. But as the series went on, I couldn't believe his transformation. He became a spiritual leader more than a warrior. It was an incredible journey for him.
But my favorite character would be his counterpart, Londo Mollari. Like with G'Kar, my initial assumptions of Londo were that he'd be the lovable trouble maker. I knew he'd be doing some mischievous, behind the scenes machinations, but I never in my wildest dreams could've imagined the hand he'd play in an attempted genocide. Watching him go from comedy relief to world destroyer to someone trying to redeem something of himself was quite the journey itself.
So many stories, so little time.

G'Kar. The pilot suggested that he may be an antagonist. As the series developed he evolved into a noble and much respected character. He also had some of the best dialog. My favourite is;
I believe that when we leave a place, part of it goes with us and part of us remains. Go anywhere in the station when it is quiet, and just listen. After a while, you will hear the echoes of all our conversations, every thought and word we've exchanged. Long after we are gone, our voices will linger in these walls for as long as this place remains. But I will admit that the part of me that is going will very much miss the part of you that is staying.