What is the simplest organism that was observed to learn: change its behavior permanently in response to some event/stimuli in a way that this change persists even if the event does not happen again?
I don't know if it is definitively the simplest, but the simplest organism with a nervous system that has been studied extensively is probably C. elegans, which is definitely able to learn.
Some basic learning abilities have also been shown in amoebae.
Ardiel, E. L., & Rankin, C. H. (2010). An elegant mind: learning and memory in Caenorhabditis elegans. Learning & Memory, 17(4), 191-201.
Saigusa, T., Tero, A., Nakagaki, T., & Kuramoto, Y. (2008). Amoebae
anticipate periodic events. Physical review letters, 100(1), 018101.