What country requires a visa?

Most (if not all) countries require a visa for foreign visitors. Most of them have exceptions for citizens of certain countries. For instance a UK visitor does not need a visa to enter the US and vice versa. There are some countries where a US citizen can enter without a visa, and quite a few that require a visa for US citizens wanting to visit.

It's best to ask at the consulate of the country you intent to visit, or ask at the authority in your country responsible for foreign affairs.