If you have weighed your options and want a fun 4 door all-weather car (aka, not just a performance car like a Corvette) then it honestly comes down to how much you have to spend. The new FA platform engine is more mechanically complex and uses direct injection and a twin scroll turbo to make daily usable horsepower while achieving pretty decent mpg. I would recommend 2016-2017 only because I have a 2015 and it has lots of little annoying quirks because it was the first model year of the new platform.
My Bluetooth connectivity is garbage and my driver information screen looks more dated. Plus, some people had issues with pre-ignition and engine mounts failing on 2015s. I’ve not had any serious issues and I’m at 75k miles now, so it’s not that the 2015 WRX is bad, it’s just quirky.
If you want the classic “boxer rumble” you going to have to purchase a WRX that is 2014 or older with an EJ platform engine and unequal length headers. These engines have historically had head gasket and ring land issues, but with proper maintenance and tuning this can be mitigated. The same is true for the new platform as well.
Whatever you decide on, get something you are going to love. I personally plan to upgrade when the STI switches to the FA platform after about 2-3 years of production to work all the bugs out. Best of luck with your purchase!