I have to go with Purple Haze ahead of SCM. Another contender has to be Beck's Bolero from Jeff Beck, b-side to the lightweight Hi Ho Silver Lining single.
Vanilla Fudge-- You Keep Me Hangin' On has to be a contender. The garage tracks above aren't as "heavy" but they are great.
I found the 1966 thread kind of interesting, and I got some great new-to-me recommendations! The timing of the mid sixties in interesting for this topic. By the time Zep and Sabbath got going, there was lots of what we'd consider "heavy" rock that was well known. But in the years leading up to that, much of what was evolving toward that kind of heavy was much lesser known, so it's great to be able to discover what I might have missed back in the day.