Babylon 5: In the Beginning (1998)

The first and the best of the B5 TV movies. Set ten years before the TV Series, it tells the full story behind the Earth/Minbari war. Much of it has been revealed in snippets before, but seeing it like this, in flashback, told from the last days of [spoiler], makes it extra special. Fans will know what happens seconds after the credits roll; the narrator knows... he's always known.
Why is there no Mr Garibaldi? They could've made him a wig.

NOTE: Despite what the title suggests, this should NOT be watched before the series, it should be watched after the 5 years have finished.

5 minor continuity discrepancies out of 5

Babylon 5: The Gathering (1993)

A feature–length pilot that's notable mostly for setting up large chunks of a story arc that would develop in subsequent years, although no one but series creator J. Michael Straczynski and the producers really knew back then because such long-running continuity wasn't the norm at the time.

If you know Babylon 5 already, then The Gathering is worth hunting down, despite its flaws. If you're new to the station, however, or not sure if a plot-heavy sci-fi show is really for you, it might be better to start with Season 1 of the TV series instead, even though it means missing out on some crucial back-story for certain characters. You can fill in the gaps later, but be aware that there are a number of notable differences between what was planned and what eventually came to pass.

3 changing faces out of 5

Babylon 5: Thirdspace (1998)

An ancient, mysterious artefact is discovered by some of the B5 crew while on a mission. They capture it and and moor it outside the station. The scientists know just as well as the rest of us that mysterious alien artefacts are always trouble, but they want to open it anyhow, and consequences be damned.
It’s debatable about where Thirdspace fits into series continuity, but we’re given some clues. It's set in the middle of the year 2261, so it’s definitely after the end of the Shadow War but prior the problems the B5 crew would have with Earthforce while President Clark was in power. That means it's sometime during the first half of Season 4. The opening narration will contain spoilers if you watch it before having made it at least that far into Season 4.

3 bozo rays out of 5