Tsk. So many fundy fanatics here.
Mine is democracy. Nothing can rival it for science, or economic growth, and managing unhappiness is very easy. It is the best for getting a good score, as there is more trade = more luxuries = more happy citizens.
Fundy is for those who prefer an extremely easy game without extending themselves.
The true challenge is to create a huge army under democracy, and use skill to fight a war.
I never understand why people value fanatics so much. They are quite useless IMO. Despite not having to pay for their support, they are still fodder for Stealth fighters.
War under democracy requires the use of intelligence operations, and an army from many cities. Stealth Fighters and Fighters do not cause unhappiness, so they are useful, and allow one or two bombers or battleships from each city. An airpower war works the best, and is thus accurate to the policies of democracies when Civ2 was made.
Only a small ground force is required, armor or mech inf for fast movement. A few helicopters are useful for occupation, as are paratroops.
It comes down to what your style and goals are. If you want a quick hack and slash that isn't challenging, then fundy is the go. If you want to extend yerself and score well, democracy is good. Communism is quite alright, but has major deficiencies against democracy.