Underrated in my opinion would be:
Mattias IA Eklundh
Luckily to the people who matter the most: those with a strong interest in, appreciation and understanding of music that shines in both relevant dimensions: technique and really great songwriting. These names already represent some of the finest to ever pick up the instrument.
However, in a more mainstream context, where popularity somehow seems to be equivalent to quality and people like Slash and Herman Li reign supreme. Those who worship them ,constantly underline their own ignorance by downsizing the people I mentioned. Using some of the most jelly sucking close minded rhetoric ever uttered.