It's the same for all musicians, not just guitarists:
1) Does their playing contribute to the song, or detract from it? How good were their own compositions or interpretations of compositions?
2) Do they advance the state of the art of their instrument, taking a long-term view?
Doesn't matter how fast, although a minimum level of technical competence is required. That ain't 1/64 notes at 200 bpm.
Showmanship is irrelevant, but fame helps. I've heard 1000 times about great guitarist who nobody has heard of, but every time they get a record deal I'm underwhelmed by everything except their technique. The problem with fame is that without it, it's hard to do #2.
They don't have to write their own stuff, or even improvise, but they have to communicate a relevant artistic vision. The longer they are great the better, but short-lived is OK. Randy Rhodes was great.
On the other hand, EVH was tremendously influential in 1990, but far less so in 2000. It doesn't matter that he spent the '80s playing with keyboards, his canon is basically Van Halen's first two albums. I believe history will restore some of his luster, but not all he once had. The basic problem is that most of Van Halen's songs weren't very good compositions and sound very dated now.
SRV has the same problem, plus his playing was very derivative. There is nothing he did that Hendrix hadn't already done.
On the other hand, the tragedy of Hendrix is that he was really just getting started. I think '70s fusion would have sounded very different had Hendrix lived because he would have singlehandedly done most of it, and made it popular. He would have been the guitar version of Stevie Wonder and then totally preempted Prince. Imagine Parliament Funkadelic featuring Hendrix . . .
Thus rock would not have split so dramatically from R&B and consequently hip-hop. The corollary here is that the '80s would have never happened, musically speaking. On the minus side, he almost certainly would have recorded a disco record.
Anyway, by #1 & #2 above, the greatest guitarist of the 1980s was the Edge.
Where the Streets Have No Name is the best guitar track of the decade.
If you prefer
the guitar hero style, try to pick up where Vernon Reid left off. The only frontier rock has not fully explored is true atonality.