I put all this effort into this post and it's no longer topical since the leak, oh well. Can't let it go to waste.
Miko raped Pekora is a classic
I love the idea that Mumei is a frequent 4cha nuser
However the greatest rrat is still that Aqua bullied Rushia. The idea that this happened is so comically outlandish I'd love if it actually happened.
They aren't really rrats but I think the best stories are the ones with some truth behind them:
"HoloEN is a myth". To this day no one knows who this was, my money is on Calli or Ame, with an outside chance of ID1 like Risu.
Mumei
leaking her model's general appearance and hinting at the Nanashi Mumei name.
Noctyx
being leaked a month in advance - the rrat is that it was Sonny but I haven't seen any evidence
Gura deliberately fucking with the rumormongers by posting a Strawpoll with her location set to Sweden
As for the actual rrats with no real basis...
IRyS = Avril Lavigne. It's just so fucking funny to imagine
The Mumei is wheelchair-bound thing. Similarly /who/ used to call her Lex Luthor all the time.
Absolutely no basis in truth but there was
this ridiculously made-up post about a StarsEN gen partnered with WOTC that consisted of five Europeans. At time time there hadn't even been a hint of Tempus and this post surfaced buried in a Kiara thread so it was just an absolutely ridiculous effortpost for a rrat that like 20 people would see.
e: Oh, I was just reminded of the Kiara = JP5 rrat, which was some interesting events strung together with no real connections.
I'd shove out some rrats about smaller corpos or niji but unfortunately they usually end up being true somehow, so it's not as funny.