What Controversial Pony Moment Had the Biggest Effect on the Fandom For Better or Worse?

The Derpy Hooves Controversy

Twilight Becoming an Alicorn Princess

Starlight Glimmer Being Redemmed and Joining the Mane Cast

Lyra and Bonbon Officially Hooking Up

Reformation of Discord

Reformation of Trixie

The Student Six Taking Over

Twilight's Castle Replacing the Library

Introduction of Cadance (Another Alicorn was a big deal back then)

Equestria Girls

Rainbow Dash and Applejack Offiically Hooking Up

Discord and Fluttershy Probably Hooking Up

Other

G5