I can't say there was a worst interaction, but there's two candidates for best. The first is that when he was just starting out, shittywatercolour painted one of my photographs.
The second is not just a reddit thing, but about 7 years ago I wrote a tutorial for r/fanfiction on how to use calibre to save stories from various websites. It was well received at the time, but since them ive had multiple times where people said it was useful, including one a couple weeks ago on a completely different website.