think the hardest I've done/remember was the 'complete all scenarios' (Complete all the Challenges in Global Challenge) in FIFA World Cup 2006 - worth 500G, but dammm it was a right pig!
most annoying had to be - Assassin's Creed - 'Keeper of the Lions Passant' musta gone through the map 5 + times looking for 1 flag argghhh
Going back through my cheevos I put some thought to it and I came up with a couple:
1.) Collecting all of the titles in Naruto Ninja Storm 2. It required too much grinding and 50 online wins. And 50 online wins is PAINNNNNFFFFULLLLLLL. Spammers, rage quitters, and Narutards abound made online atrocious.
2.) Super Meat Boy - The End. Just BEATINGGGGG that game was HARDDDDDDDDDDDDDDDDDDDDD!!! I felt truly accomplished once I was done.