This is purely down to the specialism of your course and where your interests are. A good tactic is if you have a broad range of interests to pick it based upon who is marking it.

If you get on well with the prof who looks after European Economics Policy and he is known for being generous with marks the that's probably a good topic area. 

Ah, I see I didnt realise dissertions were marked by my professor.. I actually thought theyd be sent off somewhere...

Thank you.