For those who find its style too off-putting or too many of its episodes uninteresting, maybe a "top ten" list would help, although it's subjective. This would be my list, going in airdate order:
"Beyond the Farthest Star": By Samuel Peeples, author of "Where No Man Has Gone Before," and striking some similar chords. Notable for showcasing the striking alien designs and exotic settings that TAS could portray better than TOS ever could, and for introducing the "force field belt" spacesuits characteristic of the show.
"Yesteryear": The one indispensable episode, D.C. Fontana's Guardian of Forever story which filled in a lot of Spock's background and brought back Mark Lenard as Sarek.
"The Survivor": By James Schmerer. A pretty strong character-driven story involving a guest star played by Ted Knight and the first speaking female security officer in Trek history. Features another really nifty alien design, and introduces a character, Carter Winston, who would be used in a couple of tie-in works later on. Also notable for featuring the only onscreen reference to Dr. McCoy's daughter.
"Once Upon a Planet": A fairly interesting "Shore Leave" sequel (though not by Theodore Sturgeon -- animation writers Len Janson and Chuck Menville wrote it) that delves into the mechanics behind the illusions and gives Uhura a strong role.
"The Time Trap" by Joyce Perry: Features the return of Kor, though not John Colicos. The characters are trapped in a pocket universe inhabited by a wealth of aliens including one of the few Orion women seen in canon. Gives Kor a female officer, Kali, who was portrayed as his mate in a later novel. The story is suspiciously similar to a Gold Key Trek comic that had come out not long before, and also bears a strong resemblance to Voyager
's "The Void."
"The Slaver Weapon": Larry Niven's adaptation of his Known Space novella "The Soft Weapon," featuring the Kzinti. Problematical as a Trek episode, since its version of history doesn't mesh with what later shows and films established; it's really more of a Known Space story with Spock, Sulu, and Uhura substituted for the original novella's leads. But it's still a good story. Also notable as the only TAS episode (and one of the very few episodes in Filmation's entire history) where anyone dies onscreen.
"The Jihad" by Stephen Kandel: Kirk and Spock are recruited onto a team with a bunch of interesting aliens, one of whom is voiced by David Gerrold. Mostly a big action piece, and not very plausible, but a lively adventure.
"The Pirates of Orion": The professional writing debut of Howard Weinstein, who would go on to be a prolific Trek novelist and a creative consultant on The Voyage Home
. A solid story that gives us our first look at the Orion pirates mentioned in "Journey to Babel," though for some reason their name is mispronounced as "oar
"Albatross" by Dario Finelli: A strong McCoy-centric drama in which Bones fears he may be responsible for a terrible plague on an alien world. Could've easily been a TOS episode, except for the exotic appearance of the aliens.
Note that this is actually a "top nine" list, because I couldn't decide which one to put at tenth. I suppose many would count either David Gerrold's "More Tribbles, More Troubles" (guest starring Stanley Adams) or Stephen Kandel's "Mudd's Passion" (guest starring Roger C. Carmel) due to their TOS ties, but neither is a favorite of mine; the former is too much of a rehash of the original, and the latter is kind of silly and presents Chapel in an unflattering light.