As usual, ditch despotism ASAP, or you'll find yourself lagging behind.
Now as for feudalism, it's a strategic choice. If you can't/don't/won't take the time to grow large cities, or if you're so caught up in a war with a close neighbour that city growth is at a standstill, then you might find the wide unit support for small settlements very useful. If you've got a large, booming empire, ditch it.
Communism is more of a hindrance than a help. Its communal corruption effect will only benefit the most far-flung communities, while pointlessly bogging down the more prosperous cities. Couple this with Civ3's atrocious handling of corruption to get an obnoxiously inefficient government.
Fascism is under-rated. Compared to other militaristic gov'ts, it is easy on the economy while still supporting hordes of soldiers. Forced resettlement is only a concern once you switch, and if you can conquer your opponnent fast enough, xenophobia won't be a worry.
However, Republic is pretty much the best. Low corruption and low war-weariness equals a high output and relatively long wars, as well as a trade boost to keep the cash rolling in.