Piazza San Marco, Venice. It was a flooded, clusterfuck of humanity and not even remotely enjoyable being there during the day during high season. Plus half of the basilica was covered in scaffolding. I know it’s the once place to see if you’re going to Venice, but not under those conditions. I hear the inside of San Marco is gorgeous, but I can’t confirm that. I wasn’t willing to walk through water to stand in line on scaffolding to view it with hundreds around me all vying for the same photo op.
We stumbled upon it once more at night (we had no intention of returning, but sometimes Venice makes decisions for you). It was much more tolerable.