How to decide what writing sample to include in a phd application? I'm finishing my MA in about a month and I'm thinking of applying to a Phd program in the US (political science). My GPA is as high as it gets and my recommendation letters will be great but I'm worried about my SOP and my writing sample. 

I don't know how to choose a good research topic...I feel like my interests are too broad and I don't know what topics could actually work...My MA thesis was closely linked to sociology and my preferred Phd field is International Relations...should I change the direction completely and go with something more orthodox for this field? 

Regarding the required writing sample...I wrote a 200-pages-long research paper on collocation of recent social movements and included an original case-study. It's written in English so theoretically it should work but my professor never corrected a single word...which is disconcerting...how do I know it's actually good? Part of the problem is that I'm studying in Italy so I can't find a professor who understands the American Phd application process or anything of the sort. 

Any suggestion would be highly appreciated since I'm navigating this application process on my own and I'm afraid I'm running in circles...
I think most of the other advice given above is sound. Let me add two things that I've noticed in reading work by first and second year european grad students who are transitioning into the US system. 

The paper you send for a writing sample needs to have a clearly identified thesis in the first or second paragraph of the paper. If you can't think of something better write, "In this paper I am arguing that . . . " Many of my european colleagues want the thesis to emerge slowly, reflectively, at the very end of the paper after all the scholarly qualifications and so on have been properly put forward. Don't do that for an American audience. Tell us up front what you are trying to say.

Second, hire a native english speaker to proofread your paper for you and really work on grammar, spelling, idiom, and style. Writing well is really hard and it's the primary job you'll have as an academic. (Your writing in the question is excellent. I'm just saying having somebody who really knows English double-check it.)  Coming from Italy, I don't think most admissions committees are going to hold language against you constantly. It is easy to create a subconscious impression of carelessness with casual mistakes in the language. For something as important as getting into a good grad school it's worth spending some money to get this part right.

Good luck.