For some reason, I didn't watch DS9 at the time, though I've watched all the other Treks and am a big fan - must have been shown at the wrong time for me or something. However, DS9 only had the dominion arc in the last three seasons, so I'm definitely in the B5 was first camp.

What I find fascinating is that JMS wrote in all kinds of escape hatches for if major characters left and of course, there's the time travelling shenanigans that are shown in season 1, but not resolved to much later which show just how planned out it was. Of course his 5 season arc was tied up in 4 seasons when the studios threatened to cancel the 5th season, and then when they changed their mind, it led to a somewhat tacked on season 5 as the main story had been resolved.

It seems like a common factor of having a static space stations that lends itself well to story arcs. The other Treks were based on exploration, so they make sense to reset each episode as they travel to a new region.