One of the best bits of kit I have used this year is also one of the cheapest, and surprisingly it comes not from any major outdoors brand, but Morrisons supermarket. This 350ml flask is, at £4, one of the best flask designs I have come across.
The flask body is of conventional design. However, rather than using a separate cup, or a screw off lid, this has a simple to use flip up lid, which can easily be used one handed. To open it just press in on the spring loaded button and the lid flips open to reveal a white plastic mouthpiece. It has a large hole for fluid to come out and a small hole to allow air in, allowing a smooth flow of liquid.
Closing it is simple, and again can be done one handed, and once the spring loaded clip has clicked closed you rotate a small plastic retaining clip upwards which prevents it from opening accidentally.
I use it with hot chocolate mainly, and find it best to pre-heat the flask with boiling water before putting my drink in, which helps keep the contents warmer for longer. A trip to Inverness on the train saw me still drinking the same cup hours later, still nice and warm.
Cleaning is simple, there are no hidden spots to retain excess liquids, except for behind the rubber seals which can be taken off for cleaning.
Robust, simple and cheap, get them while you can, as supermarkets seldom keep these things for long.