Although the Riddler is one of Batman's most formidable and most renowned antagonists, he's also a bit of a sore loser. Always impeded by a compulsive need to leave behind riddles that'll lead Batman to him once they're solved, Eddie is his own worst enemy.
As such, it's pretty difficult to pin down just one moment where he was left red-faced, let-alone embarrassed. If we were to pick a recent example, however, it would be from the finale to Tom King and Mikel Janín's The War of Jokes and Riddles, where the punchline - delivered by Riddler - falls flat.
Having orchestrated the meteoric rise of Kite Man from C-lister to A-lister in an attempt to get the last laugh, so to speak, over his purple clad adversary, Nygma found himself at a loss when the joke didn't get the desired effect. With Riddler having also murdered Kite Man's son in an attempt to orchestrate the character's rise, even the Clown Prince himself failed to see what was so funny.
As far as indictments go, that's pretty fierce. Considering the Joker's entire motif is to clown around, the lack of laughter on his behalf stung Nygma far worse than any Bat-gloved fist to the face ever could.