Last I watched: The Tholian Web, which I didn't like as much as I remembered, as it's just a puzzle-box story in which the only things that happen are:
- Kirk vanishes and must be saved
- Spock and McCoy bicker then comes to terms
- Space makes crewpeeps go cuckoo
...aaaaaand, that's it. Tholians are cool though, and Mike Minor's web effects were ingenious no-budget opticals.