Wow, I'm totally blown away with your art. I've seen all of them, and this is my favorite. Okay, I'm starting the critique.
First of all, the picture is very detailed and she had the colors of the mane in the right place. A lot of pictures of Princess Cadance have misplaced hair colors, but this one is absolutely fine. All of the colors are the right ones, (especially the eye). The shading for the portrait is phenomenal and the shining is just, well, let's just say phenomenal again. I'm very impressed with this one.
Next, the background is very serene and light. I can see that the colors of the background matches her mane colors. Very clever.
The horn, ear, and head is perfect (in my opinion) because if you compare it to most pictures of alicorns, they have the same features. The eye is also in the right place, as I said before, and the eyelashes are just so beautiful.
Overall, I just enjoyed doing this critique. I hope to see more to come from you and see more of these kinds of portraits. 5 stars for all.