Pinkie Pie

This is REALLY well done! The composition with her right up in your face like that is totally perfect for her personality. Something I'd suggest though: Her skin is REALLY pink, and judging by your previous human!pony paintings I'm guessing that's not what you were going for? I'd lighten up her upper cheeks, bottom nose and chin if I were you (mainly the cheeks though, leaving just a little red for the blush effect). Maybe even use a less saturated color for the shading to match the other pieces in this series.

I felt I had to rate you low on originality only because, well, Pinkie's not your character, and human!ponies have been done before- you weren't really working with any original material. That's not really a bad thing though, obviously original content is not the point of fanart (and there is certainly nothing wrong with fanart), so I hope you don't worry about it.

The other thing I'd suggest is to spend a little more time on the hair, her face is rendered so smoothly that the rougher hair looks a little out of place. It's well done (like REALLY well done, I envy your curl-drawing abilities), it just needs to be rendered out a little more I think. Other than that I love it, her expression is perfect and you really get a sense of form out of the painting. Also the cutie mark reflected in her eyes is a nice touch, and so are the freckles <3