The solo, usually electric guitar but not limited to, is probably why I'm an audiophile.
The tweeter is a big factor for me in getting the sound of an electric guitar just right. My gauge I guess would be Ritchie Blackmore's guitar solos. It's essential for me that the sound of his fender guitar sound raw with an edge to it. IME not every speaker with a tweeter produces this type of guitar sound. Like an early set of speakers I had used soft dome tweeter and I just could not reproduce this edge sound. It's also depends on the treble setting, which for me is +2, +1 doesn't cut it, as it loses this edge.
What makes a great solo is personally very subjective, a one time event, specific to a particular version of a song. For example: 'Loves No Friend' by Ritchie Blackmore. I've heard a few versions, but only on one bootleg version does he pull off the solo to my satisfaction, and it gives me ecstasy as I play my electric air guitar!
Other great guitar solos I can recall off hand:
Anya- Come Hell or High Water- Deep Purple
You Fool No One- Made in Europe- Deep Purple
Highway Star- Made in Japan- Deep Purple
Child in Time- Made in Japan- Deep Purple
Kokoro- Enchanted Evening- Kitaro
Inside Looking Out- Caught in the Act- Grand Funk
Look at Yourself- Live in Armenia- Uriah Heep
Immigrant Song 1972- DVD- Led Zeppelin
In My Time of Dying, Earl's Court 1975- DVD- Led Zeppelin
Song Remains the Same, whole album- cd version- Led Zeppelin
Headlong Fight- R40- Rush
Comfortably Numb- Pulse- Pink Floyd
Rebel Yell (extended)- Greatest Hits- Billy Idol
