When BSG aired, I thought it was so awesome... finally a sci-fi TV show with Star Wars quality SFX. Yet it failed to deliver good stories. So much potential. I really thought I'd find TOS "stale" after getting hooked on BSG. But as the show meandered across a number of directions, and showed so much repetitive SFX, I realized that it could never hold a candle to TOS. I enjoyed UFO and Space:1999 (to a degree), but their production values and episode plots seriously paled in comparison to Star Trek. Space:1999 had a lot of potential, what with Brian Johnston and Derek Meddings working their SFX and model magic. But Anderson once again stumbled with the premise, at first trying too hard to appeal to the intellectuals, then swinging too far in the other direction of appealing to the youth craving easy to digest entertainment (partly due to Fred Freiberger).
I think part of the reason why such a strong fan base built up around Star Trek was the very lack of alternative sci-fi TV programs that could match it.