Here's what I think: For me, it would be FW Godzilla and here's why: He was the most physically fit, he had a ray that (if you looked closely), managed to tilt the xilien mothership a few inches with only a slighty charged ray. He also managed to be extremely accurate with his beam, hitting his targets 99.9% of the time. FW is like a kung-fu, boxer, gymnast, and weight lifter all in one
. Think of it like this, GMK Godzilla is like the shotgun of Godzilla's, his beam has little heat but packs a good amount of force. He also isn't the most accurate with it either, when he took those planes down I thought of those orange clay discs that you throw out and try to hit them all with a shotgun. GMK Godzilla is like that old fat guy across the street who sits on his front porch with a shotgun, yelling at people and occasionly standing up to go some where. FW Godzilla on the other hand, is like a top of the line marksman that competes in championships, and goes hunting everyday. He is the sniper of the Godzilla's, his beam is like if you customized an incendiary round to where, it not only severly burns you, but it sends you flying several yards back as well.
With all that being said I conclude with this: In my personal opinion, FW Godzilla is the strongest Godzilla out there as of right now. GMK is insanely powerful, but he is just not as fast or physically fit to even try to touch FW or Heisei Godzilla, plus he doesn't have the best regen either, but he hardly needs it. It's anyone's personal choice though, do you like a shotgun that can take someone down in one or two hits, or do you like a sniper rifle with incendiary rounds that can take someone down in one shot.
Oh, and on a side note, Heisei Godzilla is like if you gave someone a pistol, who is extremely trigger happy lol.