Similar to the "What makes you adore Star Trek" thread, I thought this'd be interesting, since I don't think there's many people who unreservedly like every Star Trek series, and saying something positive about something you don't wholly like is more fun than tearing it down.
TOS - the entire first season, especially from around Conscience of the King through to City on the Edge of Forever, is the best single run of episodes Star Trek has ever had.
TNG - in its best moments, the utopian-humanism stuff really did shine through and make the show feel far warmer and more compassionate than a great deal of other genre fiction.
DS9 - I really like the first two seasons and the uneasy mood in post-occupation Bajor, and the Federation's inability to really get a handle on any of it.
VGR - this is probably my favourite Star Trek overall, but to pick one thing, I love how willing it is to swing between comedy and drama, often several times within a single episode.
ENT - the whole third season was a gamble that paid off brilliantly.
DSC - the first half of the second season is genuinely good, and has a really fresh tone - it feels recognisably Star Trek but with a modern twist. I also think the actors on this show are great, SMG and Doug Jones especially.
PIC - this is gonna sound like backhanded praise, but it isn't - the second season has honestly given me more enjoyment than any other piece of nuTrek. Yes, it's almost entirely ironic, but either way it's just an absolute joy to watch. Robo-Picard being run over by Brent Spiner still cracks me up just thinking about it, as well as the flashbacks to Picard's mother being locked in the attic. Literally every scene is just mindblowing.
SNW - I love Uhura, Kirk, Chapel, and T'Pring; all are extremely successful reimaginings of the original characters. Spock's writing's gone a bit wonky but you can't fault Ethan Peck, who for me is easily a better Spock than Quinto (no offence to Quinto). It took a little while, but I think I really can see Paul Wesley as Kirk.
SFA - Lura Thok. Also, the first half of the season was actually pretty solid, the back half is where it wavered for me.