Hi,
I want to do an Ll.M. but can't decide for a city.
I know the academics are more important but since I have the chance to move to any city in the world, I want it to be the best choice.
Maybe you have some suggestions.

Requirements:
good weather (most important :D), sunny and hot place most likely at the sea
I prefer a small city
taught Ll.M. not longer than a year
program language English
Maybe US but can also be another country (but I prefer to study Common law)

Thanks

The answer would largely depend on what sort of LLM you're after, really. Or is the goal just to take an LLM for the degree without caring about the subjects? If so, why would it necessarily have to be common law? Most LLMs tend to focus on more international subjects anyway, since most students tend to be foreigners who also tend to return home after. How small is "small" to you in terms of cities? Not really that many non-US places with some sort of common law legal tradition that fits the bill with "sunny and hot" with a nearby beach that springs to mind, although I guess places in Australia, Singapore and South Africa might qualify. I guess some gold coast unis and Adelaide in Australia could potentially qualify for your city size preference. 

Your best country will be Australia. There many law schools with affordable tuition fees and living cost. Otherwise, Singapore.