The guitar solo in Ride the Lightning by Metallica. Pretty much a 'classic' metal guitar solo, especially for the arpeggio part at the end and while it may have been quite groundbreaking at the time* (for a metal song perhaps), by modern standards it's a pretty laughable guitar solo.
*(was it though? I wasn't around back then, but you had the likes of Eddie van Halen & Michael Schenker and later on Yngwie Malmsteen & Nuno Bettencourt who could play circles around Kirk Hammett)
The unison in Afterlife by Dream Theater. Basically a rehash of the Ride the Lightning trick, but slightly altered to make it a little more interesting. Petrucci has so many better solos.
The rehash of the verse melody in Smells Like Teen Spirit by Nirvana. Yeah, I know it fits the song. It works really well in fact. Does that make it good
solo? I don't think so.
Led Zeppelin's Heartbreaker. It's sloppy, badly edited in and completely random wankery with no relation to the rest of the song. What the hell man. Sorry for offending people with this pick
... and there's probably more, but those are the first couple I could think of.
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Regarding Slayer; I'm not a fan of their music, but the 'solos' (if you can even call them that) are strangely fitting. Hearing perfectly played melodic lines in their songs wouldn't really make sense with the rest of the music.