The only thing that comes close to the greatest sci-fi quote from those three franchises is Luthen’s speech in Andor, otherwise this one from G’kar in B5 is one of the greatest bits of prose in sci-fi:
It was the end of the Earth year 2260, and the war had paused, suddenly and unexpectedly. All around us, it was as if the universe were holding its breath…waiting. All of life can be broken down into moments of transition, or moments…of revelation. This had the feeling of both. […] G'Quan wrote, "There is a greater darkness than the one we fight. It is the darkness of the soul that has lost its way. The war we fight is not against powers and principalities, it is against chaos and despair. Greater than the death of flesh is the death of hope. The death of dreams. Against this peril we can never surrender." The future is all around us, waiting in moments of transition, to be born in moments of revelation. No one knows the shape of that future, or where it will take us. We know only that it is always born…in pain.
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.
G'Kar is one of the greatest sci-fi characters I have ever seen. Andreas Katsulas did an amazing job with the character's amazing writing. G'Kar has so many amazing quotes, and every word he speaks is so very poetic. This one is my personal favorites.