He was never the strongest in marvel!!!!! One time he had a test of strength with Hulk where they stalemated for an hour and since then he has not shown himself to be Hulks physical equal. Thor is extremely strong no doubt but he has never been the strongest.
Agree on a certain level..
Prior to the stalemate, remember that it took Namor and the Hulk to subdue Thor. Thor was clearly shown as the stronger superhero.
After the Defenders stalemate however (and Hulk 300.. rage incarnate Hulk), I agree that Thor hasn't been as clear cut or as strong