Which 529 Plan should I choose?

If you have the basics down AND you filled all the prerequisite accounts already then you are ready to proceed.

Here are the top three 529 plans in MY opinion (highly-rated, low-cost and offer a good mix of age-based investment options as well as individual portfolios) when state income tax breaks are NOT taken into consideration:
1. New York's 529 College Saving Plan
2. Nevada (Vanguard) State Plan
3. Utah Educational Savings Plan

If your home state's plan does NOT offer state income tax benefits then the above 3 are probably sufficient for you to contrast and compare and find a PERFECT choice for YOU.

If however, your home state's plan DOES offer state income tax benefits then you should add it to the list of the above 3 and contrast and compare to find the PERFECT choice for YOU taking into account that there are state tax benefits to be had each year you make contributions to your state's plan.