yo_john is right, you need to look at your ratio and not the TrueAchievement Score.
If the game is generous and gives you 500GS for a single achievement and everybody who started the game achieved it, you will get a TA score of 500.
Therefore to determine how difficult an achievement is you must look at the ratio which indicates how many people got this achievement compared to the ones who played the game but didn't get it.
My top 5 by ratio:
Race Driver: GRID - Gotta Drive 'Em All - 115 (30) 3.84
Tony Hawk's Project 8 - Sick Highscore Run - 76 (20) 3.82
Need for Speed: SHIFT - Epic Master - Earn 20 Epic Badges - 93 (25) 3.72
Tony Hawk's Project 8 - That combo was sick! - combo over 5,000,000 points - 128 (35) 3.65
Tony Hawk's Project 8 - Hit 50% of the gaps - 70 (20) 3.48
My top 5 by TA score:
Call of Duty 2 - War Hardened - Complete on Hardened or Veteran difficulty setting - 317 (200) 1.58
Need for Speed: SHIFT - NFS Live World Champion - 266 (150) 1.77
Tony Hawk's Project 8 - You made it to spot 4! - 250 (75) 3.33
MX vs ATV: Reflex - Greatest of All Time - Place 1st in all events in the MotoCareer - 249 (100) 2.49
Turning Point: Fall of Liberty - Presidential Medal of Freedom - Complete Insane difficulty - 248 (100) 2.48
EDIT:
a few notes:
- The RaceDriver GRID achievement is glitched. That's why it has a high ratio.
- I did the Epic Master achievement for SHIFT without any of the online badges.
- Call of Duty 2 - War Hardened is due to what I said at the beginning of this post highly inflated
- Turning Point is a bad FPS but there are still some (Duke Nuke Forever *cough* for example) which are way worse
