I don't remember the first time I beat it, but I do vividly remember the first time I saw it.
I was living in Germany at the time and my parents brought me along to visit another American family whose kid's father worked with my dad. The kid had a NES and showed me SMB. Naturally, it was unlike anything I had ever seen; I still had my Atari 2600 and that was my only basis for comparison. He knew where all the secrets were in stages 1-1 and 1-2, and to this day I still play those levels exactly like he did. If I remember correctly, he also had Ice Climber, Balloon Fight, Clu Clu Land, and Gyromite. Not surprisingly, ROB was up on a shelf collecting dust. We tried getting it to work with Gyromite once, but it was so fiddly to set up we gave up.
Anyway, within a year I had more NES games than he did