Can only really comment on Australian venues as I havent been overseas.
AAMI Park in Melbourne when the Victory are playing - feel obliged to say that because I support them
Parramatta Stadium when the Western Sydney Wanderers are playing - most intimidated during a match I've felt as an away fan was when the entire stadium was doing a call and response chant led by their active support end. That's saying a lot as the Wanderers are only half way through their first season.