I found an IMDB poll for the most charming antihero and it includes both Londo and G'kar.

Would you consider either of them to be antiheroes?

To me, Londo was more of a tragic character and G'Kar was the reluctant prophet. While they both had moments of heroism (G'Kar more than Londo) I'm not sure the title antihero fits either of them.

Here's the poll in case you want to vote.

It often takes charm to win the audience over as a sci-fi anti hero. Which one of these characters do you consider to be the most charming? Discuss the list here.

Londo has 21 votes and Han Solo 3,357.

I guess the pollster's intent was really to see which character we consider most charming.