Coming from someone who has seen all of the older (pre-Discovery) stuff but only a smattering of the newer stuff:
This is such a difficult question because I think TNG and DS9 were peak Star Trek (along with movie IV) and are where I would point newbies, but both series have such off-putting starts. I grew up on TNG, so it was easy to get past the rough edges of the early seasons on my start-to-finish watch. However I hated DS9 when I started watching it, and only powered through because I was on a “watch all of the franchise” mission (this was back in 2015). Having finished it though, DS9 is now my favorite series in the franchise.
I think my recommendation for newbies is a curated list of a handful of TNG’s standout episodes, followed by “if you liked this, start TNG from the beginning,” followed by “if you liked this, watch DS9 but PROMISE ME you’ll watch the first three seasons before giving up, yes I know it’s a lot to ask but trust me,” followed by “if you’re still with me, start from the franchise beginning and watch in air order, although feel free to give up on TOS if it’s not your thing and skip to movies II+.”
I would actively dissuade anyone who hasn’t watched at least 20 seasons of Star Trek (any series) from starting Lower Decks. Even I feel like half the jokes and references go over my head.