The worst for me was in July 2013 from LHR to ALC, when we had severe turbulence all the way over France for approx. 1 hour. Flew through very thick cloud and its the only time I have heard the Captain say "cabin crew take your seats immediately".

Just as well they did as we bounced around a lot with lots of people screaming. Mrs Royalscot just clung on to me with a grip I have yet to experience again and she flies quite a bit.