No one want to give away their secrets - when you start a blog you should make it about something you know and like -
Do some research on the top paying adsense keywords and see how far they interest you. Then take a plunge
Here's an idea... Check out Google Hot Trends, and get some ideas about what people are searching for. Most searches these days are on various celebrities...but you'll find other stuff as well. I always use it when I need to get creative Hope that helps!
Actually I would suggest blogging on the subject you are most familiar with. No use knowing which are the most popular if you are not familiar with them. On the other hand blogging on something you are good at results in good and unique content which may bring you much more traffic.