The best experience for me was two actually. Both times they ran r/place were my favourite experiences I've had on the internet. I truly cannot describe how wonderful the experience was, as well as how interesting it was from a sociological or anthropological perspective. People coming together to build and destroy art by placing one pixel at a time. It was also a cool representation of resource scarcity. At the beginning, everyone was able to build what they wanted, but as time went on and space came at more and more of a premium, the wars over space became more and more intense. I'm done with Reddit (haven't touched it since the start of the blackout) but if r/place ever comes back, I will take part in it until it's over.
Worst was briefly getting brigaded by a toxic community mass downvoting and commenting toxic stuff on my recent posts.