I think Space: 1999 still looks believably futuristic for example. (Aside from the fact that it's now set in past! - but the design look is fantastic.)
Unfair comparison but I'll go there anyway, - 2001 A Space Odyssey - effects/ships/uniforms etc. still seem up-to-date even some 40+ years later.
Space 1999 doesn't even come close. Now before you say it - yes one was a major motion picture and the other a TV show but just sayin.