John Lord was incredible. His organ tone was the balls! Who else played a B3 through a stack of Marshall's then or now? And really, it was John that made those riffs so beefy, not Blackmore. I love the riffs but when John drops out here and there on DP songs it's amazing how small the sound got. Try Smoke On The Water without him and you'll quickly realize what a badass he was!
Not to take away from Ritchie at all...John just added so much. As did Pacey, Roger and don't forget Ian and Coverdale and Hughes! So many good iterations of the band and Space Truckin' is one of my favorites of all time. We used that as our opening song back in '05-06 when I was doing a 70's tribute rock thing called ALIVE! Even the younger kids that didn't know Deep Purple loved that song every show.