The traditional cup is about as simple as it gets! There are only seven folds. It is a bit of a test of your folding accuracy though. If you’re reasonably accurate you can actually use it & drink out of it without water pouring out of the bottom. Probably only once or twice before the paper gets too wet and falls to bits. You can use just about any paper for this one, although something reasonably robust might be a good idea if you don’t want the cup to collapse into a soggy pulp as soon as you put water in it. Paper that is coloured differently on each side isn’t necessary, but I think makes the cup look more interesting. This model takes only a minute or two to make.
My advice Fold slowly, drink quickly.
Difficulty: Very simple
Time to fold: less than five minutes