I always had Gilda as the anti-Pinkie Pie.

Flim & Flam could be anti-Applejack.

Fluttershy needs a villain still.

Gilda would probably be anti-laughter. She only enjoys pranks because she likes being a jerk, and when pranks are pulled on her she can't see the fun in it. We haven't seen Weird Al Pony yet, and I have a good feeling he's just a good-natured rival rather than a minor villain.