I picked up a lovely flask in London about 15 years ago. We'll say it's skin-wrapped stainless steel. No idea what brand, but I've never seen a prettier flask.
I rarely, rarely use a flask, though I love them. The only thing going for them is convenience. The shape makes them hard to clean, so they can affect the flavor of whatever they're carrying. And there's a stigma attached to them and people that carry them, at least in the social circles I've lived in.
Most situations that call for a functional flask, you're probably better off with a cheap plastic one you can ditch when it's empty. You don't want to get nailed at the concert with your $100 unicorn-skin-wrapped flask, and if it's for outdoors stuff plastic is lighter. You can also buy a new one as needed, so that peppermint schnapps you smuggled into church to add some pop to the hot chocolate at Christmas doesn't taint the nice scotch you want to enjoy at the end of a hike in the woods.