My favourite (I have a few...
) is one I picked up free at a trade fair nearly 15 years ago - it is mug shaped as in not tapered so is stable, plastic inside and out and has a click on lid. It is quite large - holds about 3/4 pint. Throw a tea bag in, fill with hot water, click the lid on, get on with other stuff whilst tea is brewing..job done.
I also have an all steel version - dont like it at all. The steel liner sets up an electrolytic reaction which produces weird effects with my taste buds - this explained and demonstrated to me by a Phd in Physics. Its a bit like the old kiddy trick of putting a PP3 battery across your tongue and "tasting" the electrical current. I do use it, on occaision, as a pot with built in cosy for porridge and the like.
Father Christmas brought me a Stanley version of my favourite - the top is screw on and when tightened down gives a leak free seal. Havent used it fully yet.
It was bought from TK Maxx for £3. I have seen normal tapered ones in TK Maxx for £1.