Jinzuku over on Hyjal has a fun idea: list your top five toughest and easiest raid bosses. C'thun, M'uru, Kil'jaeden and Sarth 3D are appearing on most of the lists -- the old Four Horsemen and even C'thun's trash are also getting called out by Bornakk. Personally, I haven't raided much of the hardest content, but on the hard side, I'd have to say that General Rajaxx gave my guild a rough time, Ragnaros didn't go down easy, and Twin Emps didn't play well with us (or a lot of other guilds, either).
Easiest? Chess (duh), I always found Baron Geddon to be pretty easy (though no less fun), Attumen the Huntsman, and Venoxis was a knockover, too. But as you can see from the thread, people are all over the place -- some of the hardest bosses in the game for some were simple for others. And while some guilds fly through content, others can bump their heads on bosses for quite a while. I wouldn't put him on the hard list, but I know a few guilds I've run with had quite a bit of trouble with Moroes while they were first starting out.
And it's no surprise that most of the hardest bosses in the game came near the end of expansion cycles -- AQ40, Naxx, and Sunwell. A few people in the thread predict that we'll eventually see the Lich King on these lists, and given that Blizzard goes tough when you get a couple content patches into an expansion, that wouldn't be a surprise at all.