A great Kirk/Spock episode, and the only one of their time travel episodes that didn't feel like an excuse to raid the costume department.
Tomorrow is Yesterday wasn't an excuse to be cheap, and was actually an interesting idea (mainly because it was the first time it was done).
Looking at a list, it seems that every series had a couple of good ones (took TNG until the final episode ) and a lot of crappy ones. Two of DS9's best episodes were time travel episodes, and VOY had plenty, several of which were respectable. I also enjoyed the episode with T'pol's ancestor getting stuck on Earth for a few months.
Meh, Tomorrow is Yesterday was a fairly throwaway one. Not bad, but nothing special. The City on the Edge of Forever was
the time travel episode of TOS, and one of my most rewatched episodes.
I'm trying to remember what else DS9 had. There was Past Tense, which was ok, but nowhere near their best. If we're counting Children of Time, that was a great one. A few other fuzzy area ones that I'm not sure count as time travel episodes. There's the one where O'Brien's kid is sent back in time, and they get her back older. That was a touching episode, if it counts.
Actually, Voyager had some great ones now that I think about it. Before And After was perhaps the best episode of the first 3 seasons, similarly with Future's End (and it opened up the Doctor, so huge points for it there), and Year of Hell was one of the best episodes of all. Endgame was ok, although not one of the better episodes.
And I'd forgotten about Carbon Creek too. That was a good ENT episode. There was also Carpenter Street, which was one of their most enjoyable episodes, copying the same Spock/Kirk dynamic of TCOTEOF (not as successfully, but still admirably).
They had a lot of other bits of time travel mixed in, a lot of which were kind of forced and unnecessary. While Storm Front wasn't terrible, they didn't need to finish off such a good season with such a random cliffhanger, nor did we need yet another Trek Nazi episode.
And the aliens were pretty damn cheesy.
Oh, and there was E
2. That was a pretty good one too, although another case of borrowing ideas from other Trek, with the idea of meeting your own descendants.
So much time travel, so little time.