I rewatched the whole thing fairly recently, to mark the passing of Jerry Doyle. Some random observations:
Season 1: Doyle / Garibaldi is by far and away the best thing in the first season - other actors and characters haven't quite come into their own yet, but he's got it nailed from the start. Series 1 is probably most interesting as a 'what if?' when you know what the original plan for the series was. (B5 destroyed at end of season 5, Babylon 4 to be setting for sequel series, Babylon Prime.)
Season 2 - 3: From the first moment Bruce Boxleitner appears on-screen, the series gets a massive boost of charisma and energy. The best bits of season 2 are the best bits of the whole series ('In the Shadow of Z'ha'dum', 'The Long Twilight Struggle'), but season 3 is probably the crowning glory in terms of consistency and having a clear direction. Series 2 has some weird diversions that are never followed up.
Season 4: Brilliant season opener ('The Hour of the Wolf') but oddly disappointing as a season. It has some great moments, but something feels flat - probably the need to compress everything. JMS's dialogue was previously hammy but memorable - here it sometimes feels clumsy or repetitive.
Season 5: Very underrated. A slow burner, but some brilliant episodes from the middle onwards. ('In the Kingdom of the Blind' was a surprising standout this time around.) Ivanova badly missed though - frustrating to imagine how it would have played out with her as captain (the stuff with Byron would have been so much better.)
It's still a great series, but the lack of standalone episodes means it's not one you can just dip into, like, say, Star Trek. (The only episode I tend to come back to in its own right is 'Passing Through Gethsemane'.)