Deep Purple Mark II was too awesome for words
Most of all I liked the Ian Paice's drumming. Only Iron Maiden really impressed me as much as Deep Purple in the drumming department. He was exceptional as if he was jazzing out to Hard Rock and Proto Metal, fucking awesome.
Man, I could never transition from DP to heavy metal. Simple reason is DP could write head pounding rock and very nice rock ballads like Never Before (Machine Head). To me, the trio of Blackmore, Lord and Paice was the backbone but Ian Gillan made enormous contributions.