You kinda forgot Beautiful, Dirty, Rich but I doubt many people would've voted that anyway.
I picked
paparazzi 
I already loved her music, but the day that PV leaked (and she tweeted "stop leeking my mother****ing video"), is the day I knew she'd become a legend. It's also the day I knew I was turning into a huge fan.
I love that PV so much. Bad Romance and Telephone might've topped it, but for me, Paparazzi started the legend we call Gaga.