My hardest achievement is finishing those fighting challenges with 3 stars in Batman Arkham Asylum 

Modern Warfare 2 Star 69. As I did them all alone.

I don't really go for the hard achievements but me and Call of Duty's Veteran difficulty have an awful relationship.

All Platinum medals in PGR3 and PGR4 (including the Platinum Plus DLC). Also completing Call of Duty 2, 4 and MW2 on Veteran. I don't know which of these is the hardest one, but I sure hit restart a lot of times...

I don't have a 360 but I have a PS3 and the Dead Space trophies are the same as the achievements. Beating that game on impossible was one of the hardest things I've ever done. 

" @Bennyishere: I heard those things where impossible lol wow "

Nothing Special in L4D1. Played with a group of friends for several months with all of us trying to get it. I randomly attempted it by myself one day on got it. Two of the three friends I played with ended up getting it the same way eventually. The other dude never ended up getting it, haha. But goddamn, that was a huge weight lifted off my chest that day...
 
I also have all of the achievements in Geometry Wars. Some of those were insanely hard...like getting a million points without dying. I couldn't do that again if my life depended on it. Still not entirely sure how I did it then, either. That game destroyed my life for awhile.

Well, I got gold in every race in Forza 2, but that was more tedious than difficult. I guess beating Devil May Cry 4 on Hard was pretty tricky.

" Well, I got gold in every race in Forza 2, but that was more tedious than difficult. I guess beating Devil May Cry 4 on Hard was pretty tricky. "

Probably any of my super rare World Of Warcraft achievements. I think there are probably about 500-1000 people in the world who have them, and as far as I know only about 10 or so people on Giant Bomb for some and less than 5 on some others.

" @Elazul said:

" Well, I got gold in every race in Forza 2, but that was more tedious than difficult. I guess beating Devil May Cry 4 on Hard was pretty tricky. "

I'm working on Gold in all races in Forza 3 now. 969 races. I have played this game 200 hours soon. I'm only three endurance races left (so approx 3 hours). It will definitely be the achievement I refer to when I want to brag about what an achievement whore I am... "

My five rarest are all from Red Faction: Guerrilla, but the most difficult one would either be all Jiggies in Banjo-Kazooie: Nuts & Bolts, or all Guerrilla Actions in Red Faction: Guerrilla (due to a glitch I had to start over twice when I only had five or so left out of 100 or so).

I remember doing bloody palace on DMC4 without dying and getting a blister on my hand over it. Wasn't hard to much i enjoyed it. Gears 10k achievement as well..which wasn't hard either as i enjoyed that as well. :P
 
I don't do them if there not fun really. I love a challenge but it's got to be in a game i enjoy...I couldn't grind out 10k kills in a game i hated I'd give up after 100.

my rarest achievements are all from Guitar Hero games, with the top 2 being  
100% the Final Countdown Solo 
and.. 5* all songs on coop on expert career in gh3 
 
The hardest one i have however will probably be getting the elite license in Burnout Paradise because it seemed to take forever! 

Gotta be Mile High Club for COD4

" @SneakyPenguins said:

" @Bennyishere: I heard those things where impossible lol wow "

dammit andrew. posting. "

" @Bennyishere: I heard those things where impossible lol wow "

Probably Nothing Special in L4D 
Man it took so much of my time and when I finally got it, I admit shedding a tear or two and jumping around a bit in disbelief. 
But dammit son, I GOT IT.

hmm, probably beating CoD2 on veteran, or possibly one of the TF2 achievements that are ridiculous. 

I'm more of a trophy man so whatevs. 
 
According to my achievements I'm the only person on the site to get the superstar trophy in Mirror's Edge, but that's probably just cause I managed to sync my account before trophy support didn't work. 
 Apart from that, probably the Challenge Expert achievement/trophy, that was quite annoying to get, (pretty close with the SSFIV equivalent, 98.83%!) 
 
Also I just got the Final Fantasy XIII Platinum trophy, that was pretty challenging.