for me ( Mile High Club ) is a nightmare.

Halo 3, Double Kill with a Spartan Laser in a ranked free for all playlist.
And I got it... along with 1000/1000 gamerpoints.

yeah that was pretty tough.

How about seriously... from Gears of War killing 10000 people is insane.

I think the hardest achievement is Serioulsy from Gears of War. I still don't have the achievement.

For me it's the Saint Achievement from Dead Rising. Managing large groups of survivors is no easy feat, especially before you get the book from the cult leader.

I did managed to survive, but it was on PS3.... I got a chopter and flew into Alderny after unlocking algonquin and then went and flew around for a bit before landing on top of one of the taller buildings and it took a while for the cops to turn up

Treasure Trove from Lost Odyssey.

Get every single treasure chest and hidden item in the game. We're talking a 40+ hour RPG with items hidden in every nook and cranny of it's 100+ areas, and no in game indicator or list to help keep track of it.

easy achievment for me...

Yeah I've done COD4 on Veteran for Mile High Club and got all 1000 :)

I haven't got it yet but possibly the toughest achievement I'm putting the work in for is in PGR4, to complete the downloadable Platinum Plus chapter. It's only 5 events, 3 of which I've finished. There are two street races though which border on the edge of sheer impossibility and for all my efforts nothing has come as of yet!

I got sick and started working on Forza 2... now every time I go back to PGR3 or 4 I can't play them because the requirements for driving are so different! :\

I will never be able to find all those damn flags in Assassin's Creed. There is just too many of them, and I'm not willing to spend that much time looking. The hardest one i managed to get was Steppin' Razor, since I am horrible with the sword in Halo 3. (Just in case you didn't know, that is a triple kill with a sword)

EDIT: Actually, the hardest one i got was the Mongoose Mowdown achievement.