Depends what you exactly consider "Events".
If stuff like this counts, then the lore updates with extra pages for Zaun and Piltover with short stories to celebrate the release of Camille and Warwicks rework were great.
If these don't count and you just mean stuff that gave us game modes/loot, then:
The game mode was incredibly fun for me (also I was lucky that 2 champions I really like were featured in it with Jinx and Ziggs) and beating the final mission was one of the most satisfying experiences I ever had in this game. I also really liked the augmentation system, upgrading the skills I used so many times in completely new ways, opening up crazy and new builds.
I am a huge fan of Ghost in the Shell and the skin line Project represents it really well in my opinion. I liked that we got a game mode in that cyberpunk style and hope we get something similar when Project is returning this year again.
I really enjoyed that they made such a big Bilgewater based event out of this, giving us new loot (Gold Krakens) and missions featuring different champions from Bilgewater. I think they should continue to do stuff like this for the other regions, giving us updates on champions, missions, and loot with the release of a new champion from that region. I am biased, I know, but my favourite would be a Chemtech based event with the next release of a Zaunite.