HIs sense of melody separates him from the other instrumental guitar greats for me. He has the chops but always serves the song's melody first. I don't get bored within two songs of listening to technique flash quite like Vai and Petrucci, etc...
He's got mean jazz/fusion licks in the pocket too, playing with Simon Philips and also 'The Reddcoats' amongst others. From all accounts he's a genuinely nice guy too and happy to always talk gear and guitars. I'd love to see him live one day.
I love his fat and chewy tones from clean to mean. I find the other guitar greats have their own set clean and full distorted tones and tend to just switch between those main two, Andy has some great in-between flavours too and isn't afraid of using his volume and knobs to change it up, something I don't hear as much in others in that arena.