I'm not qualified to render this judgement. I've got the time in to make an educated guess, as I've been listening to heavy metal since the mid-1980s, some 35 years I'd guess. But what I lack is the required breadth. I'm not a big fan of death metal, or black metal, or doom, or some of the other peripheral subgenres, and so can't speak to any solos that might exist in these far-flung corners of metal. Nor was I ever a fan of the true guitar virtuosos. I admire and respect the craft of the Steve Vais, Yngwie Malmsteins, and Joe Satrianis of the world, and admit they are probably the most talented guitarists to come out of metal, but I find I lack an emotional attachment to their music that keeps me from being a fan.
Most damning of all I don't play guitar. I cannot tell you what makes one solo better from another from a learned musician's perspective, and I lack the technical vocabulary to analyze music properly.
So to make a long story short your mileage may very well differ.
But for me, my favorite heavy metal guitar solo and the one that continues to leave me speechless with wonder is Marty Friedman's solo in "Tornado of Souls." I appreciate guitar solos that don't insist upon themselves. I love it when they fit the song, take off from a logical place and return to the rhythm. Friedman's solo almost breaks that spell, but does not.
I recommend not jumping immediately to 2:10 where the madness starts to build, or 3:09 where it becomes a solo proper, 3:28 where it blasts straight up into the stratosphere and parts beyond, or 3:48 where you're like, "what the fuck?" Do it if you must, but realize that this solo works best as the orgasmic culmination of an awesome song. It's worth the 5 minute investment. His skill and artistry and sound are evident right from the electric shocks of the opening notes.
If you're a metal fan and somehow have missed this one, I beg you to
rectify that right now. For non-metal fans who appreciate great guitar work, I
realize that Dave Mustaine's voice can be off-putting, but don't let
that stop you. Just listen.