For me, there were three home runs that will always stand-out in my mind. One was hit by Frank Howard (Washington Senators) over the left field roof in Tiger Stadium against Mickey Lolich. My great uncle Ray had season tickets direct behind the third base dugout and he used to take me to the ballpark quite frequently. Although I was only five at the time, I knew I had just witnessed something special since the crowd gasped in unison, was in stunned silence for several seconds, and then murmured in disbelief of what they just saw.
Pitchers learn to hide emotions - 7/11/05
Another was a tape measure shot was launched by Harmon Killebrew (Minnesota Twins) in Tiger Stadium that struck the facing of left field roof and, in my opinion, was still rising. This shot was a line-drive that went from home plate to the facing of the rood in the blink of an eye. It was probably hit in '69 or '70 (I was pretty young back then), but it had the same on the crowd as the one hit by Howard a year or two earlier.
The final unbelievable home run I saw belted at Tiger Stadium was one hit by Dick Allen (Chicago White Sox?) a year or two before my uncle's passing, perhaps in '74-'75. Allen belted one off of the facing of the left-center field roof that was similar to the one hit by Killebrew.
Overall, the two hit by Howard and Killebrew were super-human and more special. Both men seemed to bring a certain "fear factor" when they stepped to the late (and I was a diehard Tigers fan): fans would hold their breath during the windup and delivery and each mightly swat would be greeted with a huge sigh of relief from Tiger fans when the ball safely landed in catch Bill Freehan's glove.
As far as most memorable...every Norm Cash and Willie Horton blast was a special treat for me to witness. Heck, even the onces from Gates Brown, Al Kaline, Jim Northrup, and Mickey Stanley were special too.