The third season of Babylon 5 and most of the fourth season was the best run of any sci-fi series ever, and I say this as a huge Trekker.
The fifth season was a dizzying drop-off in quality, the worst case of a great series losing it since the third season of the original Trek. Maybe even worse than the third season of Classic Trek because the charecters were much worse.
My advice is to watch the first two seasons for background and the slow building of the plot. Then fasten your seatbelt for the third and fourth seasons because its a wild and wonderful ride. Then, just skip the last season and make-up your own final season…it would be much better than the one they filmed.
One thing that does blemish even the best B5 episodes is the dialogue, which often is too awful to be believed. It’s full of twentieth century slang and anachronisms, which really annoys me. They really like to say the word “butt”…“shoot him in the butt with a laser, throw their butt into space, here come the Shadows, watch out for your butt”… The writers have absolutely no gift for dialogue, except for the aliens.
On the other hand, the plotting is amazing. Deep Space 9 had its best period by simply incorporating the idea of a sweeping plot arc instead of having episodic, unconnected little adventures.
I started to watch Crusade, which looked like it could be interesting…but it kind of tapered off. I lost interest very quickly when they brought the worst charecter from the last season of B5, nicknamed Captain Cupcake by B5ers, and made it clear that the love interest between her and the Crusade captain would be a major plot line. Uh, no, sorry.