View all In January 2021 we put 11 vacuum flasks (including Thermos flasks and the Instagrammers' favourite Hydro Flask) to the test. Cheaper own brand vacuum flasks from Wilko and Asda joined these popular brands in our line-up. We tested models costing from £4 to £43 to see whether paying more for a vacuum flask pays off. Each flask was put through a number of tough tests to discover the most durable and which kept our coffee hottest for longest. We also investigated how easy each flask was to clean and which had the best build. The best vacuum flasks Only logged-in Which? members can view the full vacuum flask test results below. If you're not yet a member, you'll see an alphabetically ordered list of the vacuum flasks we tested.