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