You should invest in whatever you know the most about. Many times people will invest blindly based off what other people say, but if you don't know what you're doing yourself, you could lose everything.
With that in mind, the real estate market is expected to jump start in 2011, meaning the time to buy should be between now and then. I wouldn't invest in a small business until the economy revives, unless the business is some revolutionary new service/product. Buying stocks generally won't make you rich over night, and there is the threat of the stock market taking another dive soon. Commodities are extremely volatile, and I would not recommend you invest in them unless you are extremely knowledgeable about the subject.
All in all, I believe real estate and starting your own business is the quickest way to riches if you do it right, but again, don't invest in anything until you've done your homework.