I love TAS and believe all of its episodes are worth watching at least once.
Yesteryear is iconic and essential Spock and Guardian of Forever context but there’s a lot to take in.
TAS has some episodes that may not be the best, but definitely provide many firsts, and as such are an important bridge between TOS and TNG other later extensions of the franchise. So, here’s my list of ‘you really should see them’ episodes and why.
The Lorelei Syndrome puts Uhura in command, undoing the damage of TOS Turnabout Intruder ;
More Trouble, More Tribbles is a ‘must see’ part of the Tribbles trilogy with the TOS and DS9 a episodes;
The Time Trap - lays a foundation for the pocket universe trope;
The Slaver Weapon - it’s by Larry Niven and brings his Kzinti into the franchise, which only Starfleet Battles game upheld until they reappear in Lower Decks;
The Practical Joker introduces the Rec Room, precursor of the Holodeck, and introduces the ‘space anomaly causes Holodeck malfunction’ trope, with hijinks naturally.
The Counterclock Incident - Robert April and time running backwards, again not the best but one not to be missed.