Hi - you ask a good question. My hard-earned advice (from my own experience/mistakes) is to pick something you are very, very interested in! Otherwise, creating content my become a real chore (or an expense, if you choose to outsource writing). Take some time to figure out what's best for you. Best of luck!

@RICH had a great suggestion. Another suggestion is to think of things you enjoy doing, things you do well or even things you want to know more about. Make a list and then use the WA Keyword Tool and type in the word or words that describe what you like to do. There are also several discussion threads and tutorials on finding your niche. These are all just guides. Knowing what you're most interested in, is up to you.

Hop over to a PLR site, like http://www.theplrstore.com and see what they "stock" in your niche, often the titles of all the articles contained within each package are a good source of inspiration. :)