Ones I studied that haven't been mentioned yet in this thread: the Liberal reforms in the early 20th century (for GCSE) and the Russian Revolutions (for GCSE and A-level).
The 1917 Russian Revolutions are obviously a seminal event in 20th century history, and has been my favourite historical topic to read about since A-level. However, I was recently reading about Maoism where it was noted that in about a year's time, "communist" China will have outlasted the USSR and therefore it could be the 1949 revolution that comes to be seen as the key event of the 20th century. But it doesn't seem to be studied anywhere near as widely in UK schools. If history is studied in order to understand where we are today, then modern Chinese history probably deserves a bit more emphasis.
I studied in great detail the history of civil rights in the USA at A-level - of women, black people, indigenous people and of trade unions & labour. It was certainly very interesting and I'm glad to have studied it, but my knowledge of UK civil rights history is pretty pitiful in comparison.