So I have three I can think of off hand.
Museo Correr in Venice. I was so disappointed. It was so long, dark, dreary and repetitive with a severe lack of seating.
The Natural History Museum in Venice. I'd loved NAs growing up but this one just made me sad going round seeing how many animals he killed for sport to hang on his walls (many now extinct/endangered).
London Zoo. Went for my 21st birthday 16yr ago. It was 36 Celsius. I melted and 90 percent of the exhibitions were empty for renovation. Price was the same though.