If I could choose my own collar, it would be a substantial piece of jewelry that not only took my breath away but everyone else's and was durable enough to wear daily, perhaps a necklace with my birthstone and Daddy's alternating all the way around it like a tennis bracelet. My birthstone is emerald, and his is amethyst. Instead of gemstones, it could be an omega necklace with a pendant which might be a unicorn since he collects them and uses one as his business logo or a Chinese dragon since we were both born in the year of the dragon.
However, I'm quite sure I'd be happy with whatever Daddy picked for me.