Most memorable turbulence:
Los Angeles-London, BA 747-200. Laker DC10 ahead of us had reported severe turbulence. Apparently caused by summer sun heating the air above frozen wastes.
Auckland-Christchurch, Air New Zealand 747-200.
Caracas-Port of Spain, Aeropostal DC9.
[On both the above two I remember female passengers sitting next to me (strangers, I might add) clutching my hand in sheer terror!]
Valencia-Stansted, Ryanair. What was most notable about this turbulence was not its severity (it was slight) but that it lasted for most of the journey. The seat belt signs, which had been switched on over Spain, were not switched off again until we were over the French coast and had started our descent.