One single bill while traveling internationally would have to be $100 as they are both high value and universally accepted at currency exchange at better rates than most other currencies. It's enough to get a cheap hotel room plus meals for a day in many places which anything smaller wouldn't always do.

In practice I usually keep one or two large notes ($100, 50 euro, 10,000 yen etc) as well as a few smaller ones of both my origin and destinations if I can. Having $10-20 or similar value is handy to exchange if I need to buy a meal, souvenirs, or for a short taxi ride during a layover.