I'm a bit more critical of B5 than you (mainly because I think DS9 was generally a lot better on the simple "craft" side of things, from writing dialogue to acting) but I pretty much completely agree with your conclusions and rankings. I think you've hit the nail on the head with the arc-vs-standalone balance being the true strength of the show. Also, the characters. Just reading your short summary of the main cast has reminded me how much I loved these people and how nicely they were written.
And while I agree the arcs could have benefited a lot from some more planning and structuring, I feel like the improvisational nature of the arc writing - the ability of the writers to spin out new unplanned story strands from often unconnected exisiting stuff in a surprising but logical way - was very much a quality in itself and added a certain natural feel to the world of the show. I don't think any show before or since DS9 has had as many successful "WTF? Oh, wait, that makes perfect sense" arc-related moments for me. In the light of some disappointing stuff that came after it (which is to say, BSG
), I'd say DS9 is one of the best proofs you definitely can write a good arc on-the-fly.