From the most recent installment of Grant Brisbee's wonderful home run stadium power rankings: A video of Mark McGwire hitting a home run where nobody else has ever hit a home run.
Mark McGwire has been an unusually active hitting coach, but it's hard to imagine a pupil who could actually put what he did into practice. Traditionalists usually read uppercut swings and low batting averages as a sign of a lack of control, but McGwire was the opposite, ignoring nearly every possible outcome of an at-bat to maximize his control over what was left. Every Mark McGwire swing, and there weren't all that many of them, threatened a 500-foot home run; he wasn't fooled, and he didn't try to fool anybody else.
There have been players his size since then, and steroids or no steroids there have probably been players this strong. But there hasn't been a swing anything like it.