I have Stanley flasks. They're ok, but very bulky, and heavy, for the volume they carry.
I was recommended to try the Thermos Ultra (mentioned above). It's a fraction of the size, for a comparable volume, although the larger version is 900cc v 1lt.
I find the Thermos more efficient than the Stanley, holding the heat far longer, BUT, heat retention is catch 22. The heat rating is based on how long the heat is retained, with a full flask, but as you introduce air space, the liquid cools far more quickly.
I don't know if I imagine it, but I'm sure once liquid is removed, it stays hotter if I keep it upright, rather than laid down. Could it be something to do with the surface area of the liquid ?