An ordinary plastic cup will do very well. Do not let the weight of your cup run away to behemoths of several hundred grams in double-wall stainless steel unless you are very certain this is how you want to spend your energy.
A long time favourite of mine, a double wall plastic cup that keeps my coffee warm. Not the lightest solution, it weighs 90 grams. Unfortunately now lost, and replacements seem hard to find. Like Colin Fletcher has written about outdoor gear: "If it is any good they will stop making it."

My current solution, a combined cup and pot. Of course you can eat solids like muesli from the cup, as well.

Comments and current findings: A combination of a drinking cup and a pot is very handy. This usually means a pretty large cup that is a pretty small pot. You can certainly drink from an ordinary pot as well, or straight from your water bottle, if you want to run a really tight ship.