I second, Swerd... besides there are a lot of Trial or Demo Programs for free. So make use of them before you shell out some money. Try before you Buy. It's your choice buddy!
If someone were to offer a "best product" for you to promote, you can be sure a hundred people will jump in and the cpc will shoot up dramatically making the product unsuitable to be promoted!
Once you pinpoint the best product, it will become a bad product very soon when everyone jumps on the bandwagon to start promoting the same item, watering down your profits especially if everyone starts using Adwords. But there are some ways to sniff out a profitable product to sell. One golden rule is this: Never sell the Top Products ie those which have 200 over for affiliate gravity. These are too competitive. Go for Gravity between 60 to 100 when you are looking for a good product to sell. Another tip is to check out the refund rate. If it is low, then this product quality is considered good. There is a set of rules which I detailed in my free report that teaches you the steps to identifying a potentially profitable Clickbank product, and most importantly, how to market the product. See my sig.