This is the best one, in my opinion. The ones where's she's cream-pied and spanked are pretty hot, but this one's just plain adorable. That little heart in the center coupled with her trembling in excitement is oddly wholesome to me.

The only thing I dislike regarding all the variations are the feet, as I find paw-pads to be much more enticing than hooves. The way Capsule drew them makes them look soft, though, so that's a plus.