I just love the quality of my small Sigg Hot & Cold One flask.
It also comes with a stainless steel tea bag strainer built inside, which you can remove, or use, which ive never seen in another flask.
Bought mine in TK Maxx two years ago, for a very reasonable £12, and even though it has a small capacity, of around two cups of tea or coffee, its the portability and quality of the thing, that i like the most.
Its over engineered, and has a very nice carrying handle built into the handle
Its very high quality, for a reasonable outlay, and comes highly recommended