My personal hatred; Sitting Bum and Abe Lying. Last huge map i played, they decided to buddy up in the New World, vasseled Monty and Cupac. Ol' Abe took everything east ( including Carrib) and Bum took everything west to hawaii ( and I didn't know it at the time). I manage to carve out a nice western empire like the good little roman I am, good number of vassels and a stong econ and mil.
So I start thinking about setting up a city in Greenland, be a good launch point into the new world ( England was a vassal and was having fun ticking off mansa and shaka) and send my expeditionary force to accomplish my Grand Vision.
As I'm doing so, my fleet runs into Good Ol' Abe, heading on a similar course as me. Yes Sir, I'll take peace, even though I can't trust you ( I had him beat on turns, and I had a GA with the crew, gotta keep up morale) I know I can beat you ( or so I thought)
just after this, war alarm sounds and SB is attacking Toku ( I'm goung huh?), Find out that Monty's established a beach head Down Under, and Cupac is playing "shove the spear" with Shaka ( I am now beginng to go "oh crap"). Abe of course, is being the perfect gentleman, and is offering me all sorts of beneficial trade deals ( now i'm being paranoid) and assuring me that we're all best buddies ( now I know i'm screwed). Long story short, that little dynamic duo managed to get a Hegonomy win through their actions. Hate those two worth a passion that borders on a psychotic breakdown.
As for others, Izzy, Ragz, Jao, and David can be a pain. Ice Queen 'Beth is dangerous as well, especially if she early pops a GM or two. As for the most underated who can be extremely dangerous; Henry the VIII. That fat bearded ego-maniac has a nasty trait combo that proves deadly if he manages to get an early jump. His usual tactic is to take out Netherlands for a staging area, then take out Germ. Once consolodated, He'll go for the French Toast and follow it up with some nice Spanish coffee, once he's had breakfast, he'll look for some nice pasta after a pleasent horse ride from all those horses he just got. I don't know about anyone elses games, but in mine, he's one big Billy Bad-Ass.