You can watch The Gathering before season 1, or probably any time after that, since it is basically the Special Edition of the B5 pilot, fixing all the stuff JMS (the producer of the show) didn’t like from the first time they did it. That said, I saw it after In The Beginning, which was basically a B5 primer, giving a bunch of the universe backstory for new viewers before watching Season 5.
Anyhow, watch the first four movies, then watch In The Beginning and Thirdspace. Thirdspace, chronologically, takes place at some point in season 4, but it’s easier just to watch it between seasons rather than figure out exactly where it should fit. Watch Season 5, then River of Souls, A Call to Arms (which I personally liked a lot) and then Crusade. Crusade’s viewing order is all confuzzled, since the series had a quasi-reset after they had produced a few episodes, I prefer to just stick with TNT’s broadcasting order for this, which can also be found at the Lurker’s Guide, as linked above.
Of the movies, the least vital ones are arguably River of Souls and A Call to Arms. A Call To Arms serves as a sort of pre-pilot for Crusade, introducing the new ship and setting up the events that lead to the premise of Crusade, but the most important bits to Crusade are recapped in “Warzone”, which is one of the most-hated episodes of Crusade, in the tradition of Firefly’s “The Train Job”.
The Gathering can also be skipped, but various conversations later on in B5 (particularly late season 1 and season 2) make less sense, but since you’re already past season 2, watch it whenever. Plus, if you skip The Gathering, you miss out on ProtoDelenn, before they had the makeup figured out.
And I’d say watch season 5. From what I’ve heard, it is much better when you can watch it at once than it is when you have a week to stew on each episode. Much of it is relatively weak (particularly the first half of the season), but much overall plot important and/or character-important stuff happens, such as the departures of most of the original characters as they move on to different things towards the end. Also, if you like Bester, there’s lots of Psi-Corps-centric episodes in S5. In short, Season 5 is the Scouring of the Shire for B5.
If you DO decide to skip S5, watch the last episode, “Sleeping in Light”, it’s a necessary epilogue for the end of the series, even if you skip the denoument of S5.