Kirk,although you'll get varying opinions on the worth of watching the Spinoffs and movies, the one movie most agree is fantastic In The Beginning (It's a Prequel). The framing story is Londo as an old man telling a B5 story to children.
Yeah, it was definitely the best of the movies and I usually watch it first when I re-watch the show.
Here's the promo from back in the TNT days. Ah, memories.