What I noted from the very first episode is that, one show in, we already knew (and cared) far more about the internal and inter-state political systems in the universe than we did re: the Federation after 10 seasons of Star Trek TOS and TNG.
First season had some growing pains, and the makeup and special effects weren't always perfect (note Shol Mayan's Minbari foam-rubber forehead prosthetic bulging forward when she lays her head on the ground after being branded by Earth First), plus there were some clunkers like TKO and the techno-organic war machine one (Ikaraaaaaaa!!), but that was more than offset by game changers, where a conventional episode structure takes a radical left turn with the arrival of the Shadows or the assassination of the president. As JMS said, failure has far more dramatic potential than successful maintenance of the status quo.
Seasons 2-4 were excellent (Gray 17 notwithstanding). Season five ran into the problem that, due to expected cancellation, JMS had crammed both the resolution of the Shadow War and the Earth Civil War into Season 4. The original plan had been for Season 4 to conclude with the end to the Shadow War, and Season 5 to be a more drawn out narrative for the fight against President Clark. With the last-minute renewal, JMS had already finished his major story arcs, and had to fill season 5 with a lot of "B" material filler.
My most hated season five arc was Byron and the telepath war. Whereas the telepath underground members shown in Seasons 1 and 2 were grungy looking character actors who looked like they were scared and on the run, Byron's crew looked like they stepped out of a shampoo model convention. I guess they stumbled on a lifetime supply of conditioner and cream rinse in Down Below.
Legend of the Rangers - seemed like going back to the well (forget the Shadows, there's an even more ancient, more evil race) like Starkiller Base vis-a-vis the Death Star. Plus, that fire control system. Yeesh.
The Lost Tales - why bring a priest all the way out to B5? Weren't Brother Theo and his monks planning to stay there for decades to do their research?
Crusade was all build-up with no resolution, due to the fact it was cancelled before it even went to air, due to creative team changes at TNT. (Very Firefly-esque) Hard to judge what it could have become - like if B5 was cancelled halfway through season 1, nobody would rate it highly.