Star Wars arguably paints the picture of an imperialist democracy better. The all encompassing Republic in which people are ground underfoot by the billions, resembles what it must have felt like for the average peasant to live in the Roman Empire, or what it does still feel like for many to live in the global capitalist empire. The notion of inter-state rivalry (such as in Star Trek), and foreign powers being a threat is less realistic, as the ruling classes of different states usually just collude. The only problem with Star Wars' depiction being that it glorifies these backward paternal institutions, so that some people watching it often leave with the impression that it would be cool.
But onto Star Trek:
The Romulan Empire has a senate, and it's politicians behave like those in an imperial democracy, as far as we can tell - spin, double talk, etc, as seen in Unification Part 1/2. They probably use the age old justification of national security to launch imperial wars, expand the Romulan Empire, and justify it to the paranoid Romulan citizenry. The Romans did as much, with their justification of the Gauls being a threat, etc.
The Klingon Empire probably operated similarily at it's height in the Original Series, as even a oligarchic dictatorship requires some way to justify it's wars to the common military servicemen and servicewomen of the Klingon Defence Force. The later notion that Klingons are an unthinking horde who love war is ridiculous and racist, and belongs in a parody setting like Warhammer 40,000, not a rational and thinking show like Star Trek.
I'm sure the Federation's opponents would also classify it as one too:
- Various original series era antagonists complained that the Federation encroaches on their empire's interests and resources. This may indicate competition for rare minerals (essential in advanced technology like transporters, warp cores, etc?)
- The terrorist leader in The High Ground told Beverly Crusher that the Federation continues to trade with his planet's government, despite them fighting a protracted war against his people's continent. This may indicate a certain degree of cynical 'ends justify the means' politics in some circles of the Federation, even if many citizens like James Kirk and Jean Luc Picard are extremely just and idealistic men.
- The existence of Section 31 as well as the militarists of Admiral Cartwright era, suggests that even if the Federation is beyond the idea of the 'rat race', and no longer exploits it's citizens as labourers, using romantic notions like work and duty, it may still have a long standing reactionary/militant undercurrent that can burst out when good men like Jean Luc Picard are not looking.
It's clear that the United Federation of Planets is a much more moral and equitable society than any we have had on Earth so far - it may even be something like a post scarcity communal society, or incorporate elements of anarchism in terms of people's rights and liberties. But it is not without it's critics. Perhaps we can assume that certain rare minerals still exist in the 24th century, not found abundantly, and that the rivalry between states is largely focused on the control of these natural resources - the Federation using culture, trade and diplomacy to achieve them. Every planet that the UFP seduces into membership is one less source of vital isolinear chip minerals for Romulan Warbirds.
The fact is we don't have a consistent picture. Sometimes the Federation seems almost Utopian (and a society to be admired) - one cannot see what problem the Romulan Star Empire or Klingon Empire would have with it..... other times, it seems to suffer from some of the darker reactive elements of current society. Probably it's institutions are less inherently corrupt than our own, due to the absence of the profit motive, wage slavery, and need or want. It can therefore never be as appalling as current states. But even a post-scarcity society where money is optional, can suffer from fear and reactionary politics, and competition for natural resources.