Worst rejection I've ever received from an agent or editor?
Once upon a writers' org, we used to have editors from agented-only publishers hold open submissions for a finite period to unagented authors. We'd post our query to the forum and the editor would respond with their asks or passes. For those who received requests for fulls, the editor would comment on what part of that query piqued their interest. Great opportunity and learning experience...until one particular editor posted her pass on my submission. It wasn't the canned "thank you but not for me" that was used on other passes. No, my query was special enough to merit a diatribe, wherein the words "disgusting" and "unthinkable" appeared. And she didn't keep her scorn limited to the work, she decided to light into me too. Something in the mere 200 words summarizing the opening of a fantasy romance had teed her off. If the editor had chosen to unload her vitriol in private via email, that would've been harsh, but I would've gotten over it. The public lambasting? Yeah, that makes it memorable.
Bonus "Worst": "No answer means no."
Back in the days when agents were moving from hardcopy queries (see Jeffe's Sunday post about the beloved SASE) to email/FDM queries, a no-good and very-bad trend cropped up among the agents. In response to the deluge of emailed queries, agents and agencies adopted a "No answer to a query meant no interest" policy. ~facepalm~ No answer turned out to mean a lot of things. Mostly that technology is only grand when it works as designed, and back then filters and private servers worked less reliably than now. Lots of queries disappeared into a void, worse, requested materials were also gobbled up by technology gremlins. Once in a while, silence meant the agency was one of those outfits that respond five years after receiving a submission. Far less often did silence mean, "I have received, read, and rejected your query." From an author's perspective, the silence policy ended up being a poor excuse to avoid the bare minimum of professionalism.