Smokeybear
Emperor
Depending on the map type and size, Diplomatic or Domination are quickest.
Science is always the easiest
Culture is the longest and gets harder with higher difficulties.
That's quite odd, since all of my cultural victories are finished long before I get anywhere close to the UN or the Apollo project. Usually done by the time I get my first sniff of the Atomic Age. And frankly, I'm not even very good at it. If you are playing a deliberate cultural game from the start, it can be much faster than the Diplomatic route.