What country is the furthest north?

If, by "country", you refer to the sovereign territory of a Nation-State, and if by "furthest North" you mean extends furthest to the North, then the answer is almost certainly Denmark, which includes the territory of Greenland. Canada (Ellesmere Island) would run a close second, followed by Russia (Franz Josef Land) and Norway (Svalbard). It is possible that there are minor islands further to the North than Greenland, which might be part of the territory of another Nation-State (most probably Canada, Russia, Norway, Finland, or Sweden), but that seems unlikely.