Ah, government discussion, my favorite subject.

For me, Fundamentalism is overall the best government. I am a very agressive player (I don't do spaceships, only World Conquest), and I usually play at Deity+x, so I have to maintain a strong military to be able to beat and thus stay ahead of my enemies.
Also, it's of course very hard to keep your people happy at this difficulty level, so it's very hard to run a stable democracy. Fundamentalism is of course great, because it eliminates all unhappiness. I don't have to build any 'happiness improvements' (well I do before I get to Fundy of course; I always go to Statue of Liberty as fast as possible, to revolt to Fundamentalism), and I can fully concentrate on building military units. Science I simply get from stealing from my dear neighbours. The income in money is overall probably even larger than at democracy, but I don't need much money anyway, because my production and unit maintenance ability is so great.
My goal is always to try to conquer the world as fast as possible, so I'm usually in a constant state of war. Sometimes, when war is not possible or not beneficial at that moment, I'll switch to Democracy for a while to develop. I always go back to Fundamentalism as soon as possible though.
When I have conquered the entire world save 1 city, I switch to Democracy of course (after disbanding my entire army), to get as high a score as possible. Civscore is not very important to me though, so Fundamentalism stays by far the best government to me.
Maybe my favorite government will change in the future though; 5 years ago I would have prefered Despotism (Civ 1 heritage

), 3 years ago Communism (but that was probably because I was a communist back then), and 1 year ago Democracy.