I use an old insulated Bodum stainless steel coffee mug, mostly because that is what I had around that fit my coffee cup criteria, and because it has proven Grumman-like unkillable.
The Bodum cup works, but it is heavy, metastable tapered \_/, and, being all metal, it cools off rather quickly.
The criteria:
Well insulated. Really well insulated; if I pre-chilled the inside maybe I could use it for (discrete) beers. Mostly I want a lingering 2nd cuppa to last an hour while I pack...