G'kar and Londo absolutely made B5. The most interesting character arcs, probably the most fleshed-out characters, and Katsulas and Jurasic stole the show with their acting. All the hardships G'kar suffered and the philosophical outlook he kept despite the anger he had to deal with, Londo's moments of self-awareness, yet he remained trapped by culture and politics... Their interaction and introspection on screen was something I always looked forward to. I need to go watch that series again.
Edit: Just started watching the series again. I don't know why I didn't remember, but the writers really started off painting the Narn as the plotting and scheming baddies. Funny how that changed over the course of the series.