This is a list I compiled after reading many comments (IMDB and YouTube), listening to a few podcast episodes (focused on ST universe), and (of course) considering what I liked best. As w/ much of episodic TV, you don’t need to watch these in order. This should be helpful to those viewers who are not so familiar w/ TOS, but would like to start watching (or perhaps re-watching after many years). FYI: I watched the (remastered special effects) eps on Netflix first 6 years ago, then also over the past 2 months (during quarantine- when stuck at home). Enjoy, leave a comment, & stay safe!
Season 1
The Naked Time
The Corbomite Maneuver
Balance of Terror
Arena
Tomorrow is Yesterday
Space Seed
This Side of Paradise
The Devil in the Dark
Errand of Mercy
The City on the Edge of Forever
Season 2
Amok Time
Mirror, Mirror
The Doomsday Machine
Journey to Babel
The Trouble with Tribbles
A Piece of the Action
The Ultimate Computer
Season 3
The Enterprise Incident
Day of the Dove
The Tholian Web
Knightleyemma 
So essentially you’d recommend more than a quarter of the episodes — that’s not bad for any TV series. I think we often think of all the schlock in that show, esp the third season, but it really had many excellent moments and they were not rare overall.
“City on the Edge of Forever” is always rated the best or one of the best and I’ve never understood that.
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For the 1st 2 wks of quarantine, I was re-watching a LOTTA Jane Austen stuff (via Fb grps), BUT then got back into TOS! It’s helping me deal w/ quarantine- honestly.
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I totally get it — there’s a channel here that shows one episode per night, each, of TOS, TNG, DS9, Voyager and Enterprise. I tune in as often as I can. But quarantine has also rather strengthened my Bagginshield addiction.
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