Aside from beating the Halo 3 campaign in Legendary mode (urgh..), the one that was too much for me (I didn't finished it) was.....:
"Little Rocket man (send the garden gnome into space" from the Orange Box.
The walkthrough here is very funny : http://www.pentadact.com/index.php/2007-10-15-gnome-quest
The gnome knows no fear. He knows nothing. !!
Otherwise, the "Goes to 11" achievement (The simpsons game) gave me a headache.
It was a pure waste of time, but since I started it, I wanted to finish it.
I believe that achievements are a better way to play: It helps you going deeper into the game or play with others.
When I buy a multiplat game that lacks good achievements (and the badges that go with it), I get mad and want to throw the game away. 
Evan Wells (Uncharted 2): I think the differences that you see between any two games has much more to do with the developer than whether it’s on the Xbox or PS3.
