Which is the best reusable coffee cup?

Waitrose says I have to bring my own cup – what should I get, and is this a ploy to stop the freebies?Every week a Guardian Money reader submits a question, and it’s up to you to help him or her out – a selection of the best answers will appear in next Saturday’s paper.My local Waitrose says I have to bring my own coffee cup from next week. Two questions: which is the best reusable cup (some are more than £20!); and is this a ploy so that we don’t go in to get