That all depends on you. How old are you, do you have time to take risk? If you are near retirement, you don't want to take on much risk as this may be money you depend on for retirement. If you are young, 20-30 are are willing to invest for the long term, you can afford more risk. ie stocks. They go up and down but over ten to fifteen years they will be your best return. Then you need to decide what kind of stock, small growth companies where more return is possible but more risk is incurred because some smaller companies go down the tubes. Or do you want bigger, more established companies that will most likely be around in 15 years but will offer less return. I like mutual funds because you get instant diversification. Instead of buying one or two stocks, you get hundreds of companies with one investment so if one does go belly up, you aren't nearly as effected as if you had that one alone. You can also invest in bonds, gold, international stocks through mutual funds which are much better at hedging the risk of all the factors you probably don't have time to watch like rising and falling interest rates, currency fluctuations, unrest in parts of the world which effect various commodities like oil. Good Luck!
