Often the hardest to see especially ones where the threat is just another quiet move. I believe that finding quiet moves is where pattern recognition pays off the most.
You look at this position and you don't see anything... it has you flabbergasted.
Now the pattern comes to the rescue.
This is the pattern now how to get there.... well where did the queen come from to deliver this mate?
The only square possible is g6. From the starting position the only square you can put the queen on that attacks g6 is d6. The answer to your agony is 1.Qd6!! and black has to give up a knight to prevent the mate.