The depends on the level of service you wish to receive. If you're looking to be independent and manage money on your own, TD Ameritrade, Schwab and Fidelity all have strong platforms. Scottrade is cost effective but they strip down services to such an extent that you can't have basic things like dividend reinvestment.
If you need advice and financial planning, you can go to more traditional wire house firms like merrill lynch or morgan stanley. They're frankly over priced and biased in my view. The premium disocunt firms like Schwab, TD & Fidelity have guidance offerings. Fidelity is the worst of those offerings loaded with reps that have to push a minimum # of expensive annuities as part of their compensation. Schwab is an ok guidance offering with internal options. But both Schwab and TD have an excellent referral program to Independent Investment Advisors that use their firm to custodian client assets.