Don't go with an idea that will put you at odds with conventional wisdom on a topic, unless you can provide irrefutable and compelling proof to support your views. The concept of original thinking is well and good, but stepping on toes is not a good path to a Ph.D. This is especially important if your thesis chair or members of your committee have invested much of their careers toward supporting the status quo. If you want to rock the boat, wait until you have tenure. It is much better to pick a thesis topic that will appeal to your committee. For instance, if your chair has written several books on Roman fortifications, you might want to do something on Hadrian's Wall. The other side of this coin is that when you are writing on a topic that your chair knows extremely well, you had better know your stuff when it comes time to make your oral defense.
Match your thesis topic to the expertise of the members of your committee. This is another effect that your committee composition will have on your thesis topic. You may have several excellent thesis ideas that cannot be done because you can't find qualified members for your committee, or those who are qualified are unable or unwilling to serve. Therefore, before making the final choice from your thesis ideas, find professors who are willing and able to serve on your committee if you do topic A or topic B. Also, don't forget to ask yourself if you have the necessary knowledge and/or abilities to do this thesis. This can mean shining a harsh light on yourself, but it's better than wasting your time.
When you are mulling over possible thesis ideas, consider them in light of what is feasible. Make sure that you have access to all the materials that you need to do this thesis. If it requires equipment that your school doesn't have, data that no one has or can get, or manuscripts to which you lack access, you need to pick something else. Don't select an idea in which you have to achieve a particular result. For instance, if you are working toward a Ph.D. with a major in archaeology, you don't want to pick a topic that requires you to focus on a particular site that you can't find or solve some great mystery. If you don't find it, you fail. And since most thesis projects have time limits, you can't just keep at it, hoping that things will work out. It's better to go for a broader general topic such as excavation methodologies or new methods of artifact dating.