The important ones to watch are:
Midnight on the Firing Line
The Parliament of Dreams (optional)
Mind War
And the Sky Full of Stars
Signs and Portents
A Voice in the Wilderness, Part I
A Voice in the Wilderness, Part II
Babylon Squared
Crysalis
There is some stuff throughout the first season that becomes important later, but much of it's scattered, and might not matter much if you skip it. There are of hints and stuff I really didn't notice until watching some of the episodes again.
What happened to Sinclair later was interesting but i thought he was a wasted character.
I enjoyed Seasons 2-4, but did not enjoy 5.
I disliked season 5 immensely, and wouldn't bother ever watching it again. The loss of Ivanova, replacing her with a character with a 'magically produced' relationship with Sheriden, none of that worked for me. I felt that the closing episode of Season 4 (when they didn't know whether there would be a season 5) was a perfect end to the series.
I was told that the series climaxed in season 4 because when the show was originally created the main writer, J. Michael Straczynski, planned out five seasons. Then the show wasn't doing so good at the end of the first season, so they asked him to cut it back to 4 seasons.