IMO Chinese are the worst - and I mean Chinese nationals. There are endless stories - about tourists letting their kids poop in public places, hocking and spitting with abandon while on tour in Europe, professionals openly farting in meetings and just continuing to talk without even acknowledging it or getting red-faced. (I was filling out an immigration form in Kathmandu next to an Italian woman when a middle aged Chinese man just let one rip. He just continued filling out his form while the Italian woman and I looked at each other in shock. I said, laughing "It wasn't me!" She said, laughing "I know!") It's so bad their government had to do PSA for Chinese tourists about how to behave among Western people.
There's someone else in this thread who has a better idea about this than I - hopefully this triggers a nice list of disgusting anecdotes from him...