up until the end of season 4, this was a meaningless question. unfortunately, from then on, it's an imperfect show.

just so we're clear, if you're looking to watch riverdale "ironically" or whatever, i have nothing to say to you.

if: you want to watch the best of riverdale, then: watch seasons 1-4
if: you want to watch everything that's not tear-your-hair-out awful, then: just skip season 6
if: you want a complete picture of all that riverdale is, then: who am i to stop you. just be warned, if you care about the show, you will be very upset by season 6.
if: you only want to see the high-schoolers as high-schoolers, then: watch seasons 1-4 and season 7