Best for plushie making:
1. Pinkie Pie, the poofy hair is great for plushies.
2. Twilight Sparkle, simple and standard (non-alicorn at least.)
3. Fluttershy, always adorable, if the mane frames the face right.
4. Applejack, though the hat is a distraction unless removable.
5. Rarity, her unique eyes and prim bearing don't come over well as a plush.
6. Rainbow Dash, it's really hard to get the rainbow mane looking right.
Most likely to become a princess:
1. Twilight, obviously.
2. Rarity, by marriage if not by magic.
3. Rainbow Dash, the most talented after Twilight.
4. Fluttershy, could see her earning it by Flutterraging to save the kingdom or something.
5. Applejack, totally the opposite of royalty, though would serve if asked.
6. Pinkie Pie, just about the last pony this side of Trixie you'd want to add to a royal house.
Best at being a princess:
1. Applejack, natural organizer and leader.
2. Twilight, she can always figure out what needs to be done.
3. Rarity, can and would drop the vanity when need be.
4. Rainbow Dash, also a leader but not so great people/pony skills.
5. Pinkie Pie, great in peacetime but she better figure out how to use cannons for something other than partying.
6. Fluttershy, could you guards defend the castle, please, if it's not too much trouble?