I have said this before so apologies................
IMHO Thermos Flasks are all much of a muchness. Personally I use the standard John Lewis one litre stainless steel type (virtually indestructable!) and I use one for the summer and two in the winter. They do a good job and I've had the same ones for years, simply because I follow simple rules to get the best from them.
First of all my flasks only ever hold hot water. whatever else I add, be it tea, coffee etc. it always goes separately - separate tea bags and separate coffee sachets of good coffee and separate milk because for me they are as much a part of my fishing as my rods. I like a decent cup!
In only ever using hot water I avoid the flask 'odour' and the water stays hot for the duration. In winter I use two simply because after using 2/3rds of the first one the water is usually cold so I can go straight to the second for a really hot cup. It works well!!
When I fill the flasks at home I always heat the flask with hot water prior to filling for the trip which might seem OTT but it is stainless steel which needs heating one way or the other so preferably NOT by you trip water.....
Yes, I agree some need a bigger flask for longer outings but I still maintain two is better than one IF heat is important especially in winter. If I want to stay for that extra hour I absolutely NEED that hot(ish) drink at the end of the day even more than food.....
The last point concerns cleaning; I don't have smell problems but because of hard water a film forms on the inside of the flask and this, in my opinion, affects heat. to deal with this I use a long nylon brush made for the job and this keeps the flask in top condition.
I have no doubt others will have an equally good fool proof method, but this way of mine has worked for years and although fuffing around with teabags. milk etc may seem OTT it is worth the effort...............The end result is FRESH tea as near as is possible!
My tipple is Assam generally but in summer Earl Grey which is lighter..
ps On night trips the standard tom soup was always good but that was years ago!