It's hard to narrow it down to only one as there's so many really great ones.

I got such a kick out of the whole Electric Rodent Regime April Fool's event thing back in 2016. A TON of work went into it but it was AMAZING to see everyone cruising around with ERR avatars and banners featuring Pichu, Pikachu, Raichu, Plusle, Minun, Pachirisu, Emolga, and Dedenne to the theme of Russian-styled military propaganda. XD And the way people played along with it was totally priceless as well, definitely many fond memories with that one! XD

And as always, Stalkers, Devolution Battle Royal, Pikachu Survivor Game, all those those were always a blast and full of unique craziness.