For the combination of unsurpassed brilliance...
#1 "Nobody Knows You When You're Down and Out" as performed by Derek and the Dominoes - Clapton on traditional lead blues guitar and Duane on slide. The most perfect guitar playing on any song.
Given what Clapton (and to a lesser extent, Duane Allman) was doing in about a 5 year period between '66 and '70, if I were in the right mood, I might create a top ten list with just him/them. But I won't.
The other extraordinary performances (in no particular order)...
"Crossroads", Clapton (Cream) -- and bonus points for the performance being live
"I am Yours", Duane Allman (Derek and the Dominoes) -- slide guitar
"Sultans of Swing", Knopfler (Dire Straits) -- both the interlude and the coda are outstanding, though edge to the interlude
"Tunnel of Love", Knopfler (Dire Straits) -- the coda
-Clapton: "White Room", "Born Under a Bad Sign", "Bell Bottom Blues", "Little Wing"
-Duane Allman: "Little Wing", "Blue Sky" (Betts also rocks on this)
-Pete Townsend: "Substitute" (from the Live at Leeds recording)
-Kieth Richards: "Sympathy for the Devil"
-Jimi Hendrix: "All Along the Watchtower"
The best recent guitar solo is Tom Morello's (coda) at the 25th Anniversary of the Rock Hall concert when brought on by Bruce to perform "Ghost of Tom Joad" with him. Watch >here<