this is something that is really too subjective to talk about really all you can do is talk technical ability, which really is not as important a part as song writing and emotional music ability. talking technical it would have to be someone like steve vai or eric johnson or other people like that and satriani. petrucci is up there too. the absolute virtuosos, when you listen to them youll know what sets them apart. my problem with these guys is they dont really write songs, they just showcase their ability. i mean i like them but that right there is why they arent ever in that good of bands that make really good cohesive music. dream theater being the cloest to an exception. i mean if you have heard john5, marilyn mansons old guitarist, solo you would be astonished. talk about being held back this man has amazing ability, as far as speed and just his comfort and ease with the instrument id say he along with buckethead are two of the best guitarists in the world. but neither of them are really showcased outside of their bands. the point being there is way more to look at than technical ability, and iommi cant play as fast as any of the guys i just mentioned but is incredibly skilled at making simple things sound good(must be a name for this ability, and whatever it is he is the undisputed master), is a cohesive part of a band, not just a bunch of virtuosos who just play together and have a total loser for a singer(dream theater), and he has had a bigger impact onmusic and also on more people than every guitarist listed in that last paragraph who is fast and good. also most listed here, but not all. probably talking influence in rock music he is probably around 4/5 or 5/5.