Man, you have to watch the one with Giant Spock ("The Infinite Vulcan")

I'm hardly a TAS expert - I've seen it all, but only once - but I think "Yesteryear" is considered the highest point of that series.

The whole run is less than 8 hours total, and there’s several episodes in there that really bridge between TOS and the movies and TNG, even if they’ve been cut down for the format.

But here are a few other suggestions…

‘The Slaver Weapon’ was written by Larry Niven himself, adapting his story ‘The Soft Weapon’, and making Kzinti a Star Trek canon species.

‘The Practical Joker’ gives us the holographic simulator in the Rec Room, and the first ‘space anomaly leads to computer misfunctions and Holodeck insanity’

‘Mudd’s Passion’ gives us Scotty and the Caitain second Communications Officer Lt M’Ress in an unintended romantic twist.

Uhura gets command in ‘The Lorelei Signal.’

‘More Tribbles, More Troubles’ is an essential part of Tribble lore.

‘The Time Trap’ and ‘The Jihad’ are great for showing off a wider range of sentient species.