Everybody is different so picking one to fit everybody is nearly impossible. Here's a good rule of thumb though. Go with the most conservative calculator. Which one will that be? The one with gives you the high net worth number that you will need in order to retire. Here are three to get you started.
http://money.cnn.com/calculator/retirement/retirement-need/
http://www.bankrate.com/calculators/retirement/retirement-plan-income-calculator.aspx
https://retirementplans.vanguard.com/VGApp/pe/pubeducation/calculators/RetirementIncomeCalc.jsf