Like as the main or only instrument in a song? Probably Spanish guitar. See "Vamo Alla Flamenco" from Final Fantasy 9. I wanna say Diablo 2 had a good Spanish guitar track too, but no idea what it's called.
The first thing that pops into my head (because I recently watched it) is Chad Smith just winging a song by 30 Seconds to Mars. https://youtu.be/HMBRjo33cUE when he played it, he just felt the music.
A bonus song that I just love listening to because of the layers is like an opera heavy metal song called Awakening the Centuries - https://youtu.be/VvYRYCIAKZo
I'm biased bc I played oboe in school band but it's true, a really great sounding oboe goes totally unappreciated. I was asked to do the solo to "Kiss from a Rose" or w/e by Seal and that had probably one of the only mainstream music oboe solos. Sadly the singer went with a different song before the show.
It is an electric one, so a bit of an advantage over wood and string. The drone string is also doing a lot of lifting. Drones make almost any instrument better
Roland TB-303. It's the synth that launched so much electronic music. Examples are legion, but one of my favorites is Psychic TV "E-male:" https://youtu.be/o2U3joQ9oFI
Generally considered among the penultimate 303 examples are:
When I think of a classical instrument, I think cello - specifically this piece Shadow of Doubt by Yoko Kanno, where it has such a brooding tone and power.
Being a synthesizer nerd, I like those most, but they have such varied tones among them and even on the same synth; 4x Atlantis by Aphex Twin probably captures one of the tones I most love, and the entire "Orphans" piece hits much of the range.
I love a lot of different instruments, but I fell in love with the Erhu after ATLA. Second to that, the orchestral strings like the violin, viola, cello, etc. Scores from Halo and Elder Scrolls franchises.
I appreciate a good bagpipe. Especially when they share a lot of tonal similarities with violins like this nightwish instrumental. https://youtu.be/yOxGlXmPusI
I love horns in music, especially the trumpet. It's why I'm a big fan of Cake, they have a prominent role for the trumpet in a lot of their songs. The best has to be the trumpet solo in Italian Leather Sofa (starts around 2:10) but they have other great ones too.
There are so many, but a hurdy gurdy has gotta be up there. Played by someone who knows more than the basics, it sounds like something a medieval king would demand played to relax.
(They’re also among the most expensive instruments out there, as the only makers today are extremely talented individual ones; no big company like Fender or anything makes them. If you see one, you know it was handmade by a master.)
I also love the sound of the Crwth. It’s from Wales, and is sort of like a regional spinoff/precursor to the violin.