Deep Purple, Sabbath and Rainbow with Ronnie James Dio are by far my favorite bands of the 70s so I really had to think on this one.
Richie brought in a whole new virtuosity with his dexterity and his interest in applying classical , medieval and middle eastern chords to heavy rock. A real game changer.
Tony on the other hand came up with an entirely new style of play and created a whole new, heavier , darker genre . Both are masters of their own style and well loved by me.
The only way I could choose between the two was to take both entire discographies into consideration. I absolutely love all of Blackmores work with Deep Purple and his output with Rainbow up to and Including Long Live Rock & Roll. After that the quality drops considerably in my opinion. The highlighted albums are some of my favorite all time music without a doubt .
Iommi , on the other hand , has not put out one single album with Sabbath that I don't love, other than Forbidden, which is definitely of lesser quality.
By virtue of this comparison my vote goes to Tony Iommi.