1) I don't know whether Michelle (or so Andy says anyway :E) Weikath is the best guitarist overall, but he has some awesome solos. As for a comping guitarist, that's not bad. Also, can do his job extremely well, too. Looks mainly bored live, even when tapping some extremely hard parts. That's all showmanship to me.
2) As a founding member the only (non-bass) guitarist who's been there all the way. Arguably the co-"founder" of power metal with Kai Hansen with songs such as Oernst of Life, Guardians and How Many Tears. Although there were power metaly songs for example on British Steel (Priest) and Balls to the Wall (Accept) already.
3) After all, he's the one who's written some all-time best songs. While especially Hansen has written a lot kick-ass material, actually way more than Weiky himself, he's never reaches the level of the song Keeper of the Seven Keys, a true masterpiece.
4) Has a lot of wider music composing ability than the rest of the guys. A really wide span from hit-quality songs such like First Time, I Can and Sole Survivor to rapid power metal anthems such as Falling Higher, All over the Nations, Born on Judgment Day (a lot of those) to the very varying "big songs" such as Revolution Now!, Nothing to Say, How Many Tears and Keeper of the Seven Keys. Also can write good ballads such as Windmill and A Tale That Wasn't Right.
Not written nearly as much material as Hansen, but I think he's, if not technically best, at least the most creative and most experienced guitarist of _Helloween_. Hansen might go ahead of him if we talked about melodic metal guitarists overally. Or I don't know, Keeper's (the song) just that good...
<EDIT>
Still have to comment on the singer issue as well. I don't think Kai is a good singer. He's become better. In the WoJ/Guardian era he sounded awful, while we already sang Land of the Free pretty well. I think he's become even better from those days. Still, he isn't a good singer in a traditional way. BUT, his snotty voice fits Gamma Ray like a fist in the eye. His shrieks are also awesome; a death metal singer would be proud. His style is so unorthodox I think he overcomes Mr. Scheepers any day. And, speaking of Scheepers, I think he's become even better, too. While his Kiske-like heights have come down for the last ten years, he's compensated that with power and aggression. I love how his singing sounds very painful these days, like he spilled blood and lungs. I very much like that kind of Eric Adams / "Ripper" Owens style he has nowadays. Just brutal. Still goes pretty high if he has to, though. The same goes with Kai, but he's just so unique. Awesome with Gamma Ray, but not a good singer at all in a Freddy Mercury way.Ralf Scheepers is a 100 times better then Kai Hansen will ever be. Kai made a big mistake when he let Scheepers go in 1994.
Roland and Kai meant business, he doesn't. 
