*sigh*
This is very hard to answer. I like all three, and I think all three are outstanding guitarists.
Janick is a master at songwriting, and I think that in the last ten years, he has really evolved to a masterfully skilled guitarist, too. I'm not overly fond of some of his 90's guitar work, but nowadays, I would say he is world-class. And I know a lot of people will disagree with me, but I think that he is also a supreme live guitarist. He is responsible for two of the three moments when I was actually 'blindsided' by a guitar solo during a gig - Powerslave in 2008, and something during the last tour (maybe Blood Brothers, but I'm not totally sure anymore). The only other moment when I had similar feelings was when H kicked off his Rime of the Ancient Mariner solo in 2008.
But I would propbably have named Dave my definite favourite before the release of The Final Frontier, simply because of his laconic and yet involving guitar work in Out of the Shadows - but now H has his Out of the Shadows moment during Coming Home. But he adds on that during Starblind, and his guitarwork there is my favourite moment on the new album in total. So currently, I would say H is my favourite of the three, but it's a very close call.