Absolutely agree with Brian and Rodders. There are... perhaps half a dozen episodes from the first season that are genuinely important to the full story arc, and are excellent episodes. There's probably another half dozen with minor events that will get referenced and built on later on that are worth watching. The rest are almost entirely standalone (though, it's worth noting, still help to build up the characters, despite the often ropey stories themselves
)
Season 2 is where the good stuff really started to kick in (especially in the back half as tension mounts properly)
Season 3 is absolutely amazing TV throughout (with a couple of poor episodes in between, though even the poor ones have subplots that are still good) with some of the best stuff in the history of the show. The 3 part story in the middle regarding the civil war being a particular high point.
Season 4 is, again, amazing stuff throughout. Some people feel it sags a little after the resolution of some big events, but it soon develops other plot points that have been undercurrents for some time and is generally excellent.
Season 5 suffers from the fact that they had to pull forward plot points into Season 4, as they weren't certain they would get a fifth season, thus when they did, they had to pad it out slightly (they also had to replace a main character whose contract wasn't renewed in time for the same reason). it still has good continuation of the overarching story, but doesn't quite reach the
sustained highs of the previous two seasons. It does have some excellent individual moments though.
Most of the movies aren't critical, though the first "In the beginning" is absolutely worth watching. If memory serves me it's best to watch after the 3rd season to not spoil plot details for yourself.
And for completeness, as I'm always keen to help promote B5
if I had to compile a 'should watch' list of first season episodes... I'd go something like this: (though this is largely from memory and a list of brief episode details right now... so apologies if I name a stinker)
0: The Gathering, the pilot movie-length whatsit.
1: Midnight on the Firing Line
6: Mind War
8: And the sky full of stars
13: Signs and portents
17: Legacies
18: A Voice in the Wilderness (part 1)
19: (part 2)
20: Babylon Squared
22: Chrysalis