Watching “The Best Night Ever”, in terms of who ruined the Gala from most to least, I’d say it’s: Pinkie, Fluttershy, Rainbow, Applejack Rarity, and Twilight.
Pinkie: Annoyed the other guests for most of the episode by trying to force them to party the way she did.
Fluttershy: I put her a little below Pinkie because she only really ruined the Gala at the end with the stampede.
Rainbow: Didn’t really do anything bad. She pulled a stunt on an innocent pony but did also catch him. She also accidentally dropped a giant statue that broke a bunch of columns, but tbf that was underestimating her strength and she did catch it to save it.
Applejack: Didn’t really do anything like Pinkie and Fluttersy, but she did trip somepony to try and sell them something. She also barged in with a giant cake to “prove” her wares despite there already being a buffet table. A little below Rainbow because while she did barge in with the cake, it’s Pinkie’s fault it went flying.
Rarity: Only really (rightfully) lost her temper after Blueblood used her as a shield. Didn’t really do anything else.
Twilight: By virtue of us not seeing her do anything.