Seasons 2, 3, and 4 of B5 were probably the greatest seasons of any sci-fi series on television.
Star Trek comes in a close second, but only due to seasons 3 through 7 of DS9 and season 4 of Enterprise, along with random episodes of TNG, and maybe one or two episodes of Voyager. TOS was too before my time, but I did, of course, love the movie trilogy (TWOK/TSFS/TVH) and ST:VI-TUC.
I enjoyed the miniseries and seasons 1 and 2 of BSG, mostly because of how dark they were, but the show went so far downhill afterwards that I often wish I hadn’t seen them.
Star Wars I never really enjoyed all that much, probably because I only saw it for the first time in my 20’s after enjoying DS9 so much.