I have used Fidelity for over 10 years. They have a wide variety of mutual funds to choose from and their online interface is intuitive and easy to understand. Best of all, they have a LOT of low-cost mutual fund options.
Vanguard is another industry leader that I trust because of the reputation it has developed. This company is completely geared toward helping its investors succeed in their retirement planning by providing simple, low-cost retirement solutions like index funds.