I know "underrated" is an overused word, but Mark is an underrated guitar player, no doubt about it, and nobody can prove me otherwise. Not only did he manage to establish the sound that is truly recognisable from a single note, but he did it without any sophisticated equipment, and not only that, he used it in the context of hundreds of his songs/riffs. For the amount of creativity, beauty and fun he brought to the table in the guitar world, he deserves very high positions in whatever lists. It doesn't even matter if he stole this from J. J. Cale or this from Chet Atkins. And so what? Who cares? You try to steal it from Mark and I'll watch how far you'll go.
I think somebody said the guitar is an instrument that is the easiest to play badly and hardest to master. Once you realise how damn hard it is to play like Mark, everybody else pales in comparison. Another problem with these lists is that they lack a million players from genres other than "rock". There are monster jazz, country, folk, and metal players. I generally hate any lists and can't care less about them, but the underrated nature of Mark's music bothers me, that's why I do what I do.
Of course, all this "underrateness" is actually exactly what Mark wants. I'm 100% sure if he wanted it, had more exposure, and pursued this goal in the first place, he'd be regarded as one of the greats. The problem is he doesn't want it, and so naturally, he is either getting forgotten or underrated. Number 100 or something on the stupid list of the greatest guitar players is pretty fair in this light. I bet he'd be more happy not to be there at all.