I have 3.
The first time I used Metronome was with my Poliwhirl in Yellow. I used it while battling Magikarp... and it turned into EXPLOSION! Talk about a bad first impression!
Much later, again in Yellow, I used the Mew Glitch to get a Mew of my own. It knew Metronome when I faced a wild Ditto on Cinnabar Island. The Ditto transformed into it, and then used Metronome... AND IT TURNED INTO SPLASH!!!!! No opponent has used Metronome against me in that game apart from that!
In Diamond, I had a Togetic with the move. I used it in a Double Battle at the restaurant at Valor Lakefront, and it turned into DARK VOID! Both opponents were suddenly put to sleep, and I won hands down. I'll add, also, that Darkrai had not been released at that point, so I learned the animation before I was supposed to!