Many hubbers are reluctant of answering this for fear of people stealing their hub topics and thus making the keywords more competitive (or worse--stealing the articles themselves).
I will say that my most successful hubs have been the ones on very specific, targeted topics. A few of my cosmetics hubs do very well (the more popular the product, the better it does) and some hubs on seemingly random topics do very well (because the keywords are very, very specific and there is little competition for them.)
Use Google's keyword tool to check out topic ideas before you spend the time to write the hub. This will give you an idea of how often topics are searched and how competitive keywords are.