For me, no one has bettered Thermos. Get a steel inner and properly insulated top (no fancy pouring mechanism - they just let heat out).

Although you are unlikely to get a cheap, slim or lightweight version compared to other flasks, it does the job - piping hot, all day, even after opening.

Forget the flask, they are all rubbish after the 1st drink, get 'Jetboil' same weight (or less) than a flask boils in one minute, always a piping hot drink

Hands down "Thermos light and compact"

I bought trails best in test, filled it at 6am and by 2pm the water was not hot enough to brew with. So I looked ont' tinternet and heard about thermos's light and compact. Mine cost less than £20 delivered which might be more expensive than every other flask but it is infinately better than my first flask.

It is that good you can still brew up with it after 24 hours. I would only keep water in it though as the lid is pretty fiddly with loads of bits in it.

I also find that a hip flask is pretty good......

The efficiency of a vacuum flask depends on the integrity (and extent) of the the vacuum within the inner shell - I find that in general, the more expensive and heavy it is, the better insulation it offers. In tests in previous years, the Japanese brand Zorujishi (? spelling) always came out tops but I've never tracked it down in outdoor stores - maybe Harrods stock it?