As people have said, TP, the Shakespearean sub-title for B5 would be 'The tragedy of Londo Mollari.' His mistakes, weaknesses and regrets are the story.
You know, I STILL don't see that. I've been hearing how "Babylon 5 is really the story of Londo Mollari" for years now, and I just can't see it. I finished a re-watch of my B5 (and Crusade) DVD sets two weeks ago, and this time, mindful of that, I was watching for it, but I didn't see it. Babylon 5 is as much about Sinclair and Sheridan (and everybody else) as it is about Londo.
JMS said that he wanted Babylon 5 to do for science fiction what Hill Street Blues did for cop shows, and as far as that goes, yes, I see that comparison, but the whole "Babylon 5 is really Londo's story" just escapes me. It seems to me to be more about the esemble as a whole than any one character in particular.
JMS says that Babylon 5 is the story of Londo & G'Kar. So I guess that's why people say it. It's also the story of the other characters but to be fair, the other characters don't necessarily have the same focus as those two. He also probably remembers Peter and Andreas more than anything.