The short answer is : The biggest one you can afford. For a gas heater, a 400k BTU will use almost the same amount of gas as a 200k heater in the end. Sure, the 400k burns twice as much gas, but it will raise the temp of the pool in half the amount of time, so the net gas consumption is virtually identical. The cost of a bigger heater isnt that much more.
Like john asked, are you looking at gas or a heat pump and where are you located.