I've never purchased one, but I always like the ones which are a mini model of a beloved character, for example, Mr Potato Head. As opposed to a random shape. The Dumbo bucket which I spotted on my most recent trip immediately grabbed my attention, but it was very expensive. I think it was the most expensive bucket that I saw. The BB8 bucket also looked good.
As wanderlust7 said, I think it would really come down to your favourite character or something which simply sparks your imagination / interest.