Worst of the worst : Paris subway, line 13, when it was on strike, in summer.
30°C (86°F) at least in there, people pushing, shouting, sweating, 25 minutes trip, no air, no room, a daily delightful moment for commuters like myself. Hopefully, this time is over ...
And also crossing the sea from Italy to Greece, at night. Like a big storm out there, people puking everywhere and me sitting in a chair on the upper desk, reading because I could sleep. I'm glad I'm not sea sick