Depends how you want to go. Best can be defined many ways, best quality, best price or best for right now.
I have 2 Bradford White NG water heaters $300, both natural vent one 10 the other 8 years installed. I expect to replace them soon. If I get 12-15 years out of a water heater I am very happy.
My parents did a SS indirect tank off the oil boiler that was about $1500 but I expect 20-25 years out of it.
The Polaris is interesting. High hourly flow capacity and low heat loss. Was going to replace a floor furnace with a water to air exchanger fired with the Polaris.http://polariswaterheaters.com/support/manuals/Res35-50.pdfhttp://polariswaterheaters.com/support/specifications.aspx
If you have power failures/issues you want natural vent, if not go for condensing.