Fun fact: the first computer game I worked on professionally was "My Little Pony and the Runaway Rainbow" for PC. The game used the old style ponies, but Rainbow Dash was in that game! I got to help model her crazy rainbow hair! "Make it 20% cooler!"
If I'd have to pick this would probably be my least favourite from your humanized Ponies. The painting in itself is great, but I just can't feel the right Rainbow Dash vibe from it, if anything I'd expect Dash to be and look more... Wild. Hair probably bugs me the most, I certainly wouldn't expect Dash to groom her hair, or if so then certainly not this much, that would be probably one of the most distinct things indicating her carefreeness and toughness. Another thing would be her expression, some wild smirk or something would be in my opinion more fitting, but expressions are pain to draw properly so I'm not gonna argue here. Not really sure if you intended to make her less tomboyish or if it turned out like that on its own, but I feel her tough attitude (even tomboyness if you want) is one of her main traits, one of the most important and distinctive ones which I can't really imagine Dash without, and I can't feel it from this picture.
But just my opinion, like I said, the painting itself is still amazing piece of art. Continue the good work, and sorry for the rant, I don't know how to write to the point.