[Van Hagar]

For me, the most underrated rock guitarist is Gary Richrath, the original lead guitarist for REO Speedwagon. Go back and listen to their early live albums, and check out the instrumental “Flying Turkey Trot.” The dude was sensational.
Another guy that doesn’t get his due is Drew Abbott who was the longtime guitarist for Bob Seger. His stuff was just so clean and clear.
For other genres (although they do/did a lot that was rock) I love the guitar playing of Prince and Keith Urban.