Ah, can't forget Jimmy Page, of course!
Stairway to Heaven is obviously obligatory guitar material for any guitar player, legendary needless to say.
Still... aren't we slipping sort of into Artist and Album territory here as opposed to actual Solos?
Which sort of makes me question - what do we all call a legendary guitar solo? Is it, say, a solo that is bound to go down in history or a solo we just like? And also, it seems people have been mentioning more than one musician for a solo... is it really a solo if you have two guitarists playing, working together? Wouldn't that make it more of a duet, not a solo?
In that case, a most worthy & legendary electric guitar duet is, by and far, Robert Fripp + Adrian Belew. Just because there simply isn't anyone in the world who does the interlocking guitar stuff the way they do.
But, I digress. More to solos...
Here's some of my favorites, lifted directly off of the "100 greatest guitar solos" list by Guitar World magazine...
Jimmy Page - Stairway to Heaven (listed #1 in the survey, no less)
Jimmy Page - Whole Lotta Love
Jimi Hendrix - All Along The Watchtower (shoot... forgot it last time around)
Jimi Hendrix - Voodoo Chile (my personal fav)
Don Felder/Joe Walsh - Hotel California (again, one of those obligatory things)
Ritchie Blackmore - highway Star(dude, you cannot underestimate Ritchie's influence... there's a whole school of guitarists who took after his style)
Brian May - Bohemian Rhapsody (just to solidify the Queen example there...)
Brian May - Brighton Rock (Funky use of simple delay... love it)
Mark Knopfler - Sultans of Swing (I love this guy, he's a fun guitarist if there ever was one)
Joe Satriani - Surfing with the Alien (oh wait, Propyro mentioned that one... well, it deserves a double mention

)
Frank Zappa - Zoot Allures (ah, yes, that one too)
Artists not mentioned with concrete examples but worthy of mention - Stevie Ray Vaughan, Yngwie Malmsteen, Jeff Beck, Adrian Belew, and maybe even Tom Morello.