Memorable to you. Feel free to use whatever definition you want for that - iconic, unforgettable, most melodic, best suited to the movie/TV show, whatever you want. I've tried to include as much as I could on the list. Some things I separated even though they were on the same soundtrack because they're generally recognizable separately (eg. Ilia's Theme vs. The Enterprise). If I've forgotten anything, my bad!
Personally, even though I love Goldsmith's themes, Eidelman's TUC theme sticks out for me. His theme is bombastically unique, suits the movie incredibly well, and stands out even more because it's sandwiched chronologically in between a bunch of Goldsmith and McCarthy works.
What if you could only pick one? If the world was coming to an end, and we were building a culture capsule for humanity to send off into space, and there was only enough memory for one theme to represent Star Trek amidst the entirety of human knowledge?
My first choice on instinct is the TMP/TNG theme. It is as iconic as the TOS theme and instantly recognizable. It's music thats so good, they used it twice! For me personally, my favorite was Voyager. The music combined with the visuals for the credits is an audiovisual treat.
Let's hope that one song ain't this one: Spoiler TNG theme. Yes, it started as TMP and has a proper majestic feel, but it felt so out of place for TMP/TOS at the time -- what happened to Courage's TOS iconic score, which is just as cinematic? (TFF also using the TMP theme feels just as out of place.) Somehow, the TMP motif just worked better for TNG and became properly iconic as a result. Runner-up: TWOK, which is Horner's re-scoring of Courage's original theme and to great effect.
I voted for ST: TMP/The Enterprise/TNG. Sweeping, iconic, unforgettable. That said, my favorite is the Voyager theme. If this were a poll for "which music gets stuck in your head the most?" then that distinction unfortunately belongs to the infernal "Where My Heart Will Take Me."
I used to love "Ilia's Theme" until I recently bought the La La Records 3-disk release of the TMP extended soundtrack, and made the massive mistake of listening to "Star Beyond Time" Ruined forever.
I had to go for DS9. Both versions are simply amazing pieces, if you ask me. And although the memory is bittersweet, my exgirlfriend and I had decided to use it as our weddingmusic. It's that good.