Sorry, it was the first image which popped into my head. Prolly not a good idea to keep this in one of your tanks or near your house for that matter. Again, sorry I'm an idiot
Of ones that can be easily kept submersed? IMO that's probably C. usteriana x walkeri. I got one from Aaron a while back, and I've only seen two other people that have it, and both got it from Aaron.
C. x timahensis is an Über-rare emersed-only crypt. According to Ghazanfar, it's only found in one small pool in Singapore.
I've seen x Timahensis on aquabid. No kidding. So the UST X WAL hybrid probably wins.
Come to think of it, so have I, but it was being sold by one of the Malaysian sellers. Ghazanfar is the only person I know of that has C. x timahensis in the U.S., other than maybe AaronT.