Bit late to the party, but... most annoying pax I've ever encountered was on route to OTP who didn't like using headphones. Despite having some, she watched whatever irritating TV programme she had on her laptop at full volume.

I asked her to use headphones and she simply ignored me and then went on to kick the back of my chair for the remainder of the flight

Second most annoying pax was being surrounded by a group of "lads" (in their 40s) on a sex tour of some remote bit of Russia who spent the entire flight to Moscow discussing what and where they were going to buy sex. I complained about this one, but they didn't do anything...