I don't know that I can pick one favorite shade in toenail polish. I like the entire red-pink-violet spectrum, but any eye-catching color works great for me. A lot depends on the lady's skin tone, and what she is wearing.
One of the damsels I play with wears Vixen by Revlon on her toenails. It is a deep, dark red, with subtle hints of violet and brown. It looks fantastic on her.
I like and have just about every color out there and continuely buy more.
I try to determine my favorites by nail base length, skin tone, nail plate pigments and her personality