This seems to be more of a TOS vs TNG thing but just to mention DS9, VOY and Ent that never reached TNGs level of success. The most recent treks have had a lot more competition and especially with Ent after 18 years of continuous trek it just wasn't new anymore and with trek carrying a heavy stigma it was never going to attract mass audiances even though seasons 3 + 4 where very good.
As for TOS, I always liked TNG more personally, the cast was a lot better developed, was a good ensamble and really explored a lot of science fiction and the humanity behind it. Though I think the films where definately best done by TOS.
Would be interesting to see how things had ogne if TOS gone for a whole 7 seasons and had the equivilent budget TNG enjoyed... but then had that been the case the last 40 years of star trek would have gone a lot differently!