I was inspired by this question: Which (non-Earth) planet appears in the most Star Trek episodes? to ask the same for Babylon 5.
I'll also adopt the same rules:
- "appears" means a scene in the episode is actually set there (mention of a planet is not sufficient)
- versions of the same planet in parallel universes count as the same planet for counting purposes (not sure if this will affect the outcome in B5)
- both Babylon 5 and Crusade are included, but not the movies
Off the top of my head, I would probably say Centauri Prime or Mars, but I'm really not sure!