If you have multiple ideas for adsense sites, the best advice is probably to throw ALL of your ideas up. Create a simple site for each idea and see which one delivers the best ROI. It's highly probable that you'll get some kind of income from each and every site, but some will undoubtedly outperform others. Then you can focus on developing those sites further, and perhaps develop other monetization options for them too.
In basic terms though, if you want to prioritise, look for the one which you believe addresses a niche which is most likely to be hungry for information... OR the one which is most starved of resources (ie, fewest direct competitors).