"Hedwig" at the Jane Street Theater in the first or second week or previews, when it was this little show booked for a 4-week run that no one was paying much attention to. I only went because I had a teenage crush on John Cameron Mitchell, had no idea what to expect, and had my whole world changed in a tight 90 minutes. (Over the course of the run, those 90 minutes became less tight, but in that early preview, Mitchell stuck to his script.)
This also made me think of a somewhat different scenario: seeing Cynthia Erivo in "Color Purple", having been told she was amazing in it, and waiting for most of the show for her to do her "amazing thing", and wondering why everyone was raving about this thoroughly average performer... and then she sang The Song and I lost my mind. Sometimes, even knowing there is an iconic thing can be amazing (it helped that we hadn't actually seen or heard her doing the song on YouTube or late night TV).