The longest to date generated by ThePiachu's Vanity Pool & thread btw is 9 chars after the initial 1 for 1ELiZABETHdzGTuQeg2RnCcWBMVo3ySfTz that I'd requested as a gift for someone.
But not the "most difficult" - it is twice as difficult as a normal 9-letter because the "i" can only be lower case. A nine-letter with two "i"s and an "o" - 4x more difficult than that...Proof of 10: http://blockchain.info/address/1EXEcutaBLEbJEV8BYX4CPbzvb5131zXnQ; not very weird though.
I think "1Fake" wins for completely random happenstance with unintended outcome.
Mine is long, didnt know it was impressive til now though 
1JASiNZxmAN1WBS4dmGEDoPpzN3GV7dnjX
1JASiNZxmAN1WBS4dmGEDoPpzN3GV7dnjX
^ I have the private key, got it from vanitygen
However if you were searching for "jasin", the rest is just something that randomly looks like something, like mine in my sig could be "1bitcoinmeet"...