For me, the hardest loss is to a worse player. I recently blundered - fortunately not too badly - and went down a Pawn and bad position to a player 400 points below me in a 3-day/move tournament. I worked very hard on that game, compared to my other games, because I did not want to lose or draw. I complicated the game, worked to gain the initiative by sacrificing another Pawn, then eventually won.
On the other hand, I've too often made a bad move against a player rated well above me and paid for it, but didn't feel as bad!
Well seriously , there are times when you are a mess after the opening but somehow manage to solve all your problems, but then you manage to lose a drawn endgame. After spending all that energy only to lose the game in a theoretically drawn endgame. So need to work in that endgame!