depends on the utopia. in fundamentals, communism is best overall as we're all equal. but in reality it's utterly impossible as a concept due to human nature, specifically greed. so in a utopia where everything's perfect and there's no flaws i'd say communism. if it's a 'utopia' but humans are still 'human' then democracy is meant to be the 'fairest' way, whilst monarchy/dictatorship would be most efficient at getting things done.
actually thinking about it...i'd say in a utopia that dictatorship would be best, as long as that leader was the perfect leader in such a utopia. no one feeling beneath others and he/she's obviously doing a good job as it's a utopia so why question their rule?
If you could combine two animals, what animals would they be? what would the new animal be called and what could it do?