You rang?
The worst achievements, in my book, are the rare online achievements:
Ghost Recon Advanced Warfighter has three - you have to be number one on the solo, team, and universal leaderboards

College Hoops 2K8 - Qualify for the Pontiac Virtual NCAA® Final 4® and complete at least one game in the tournament. This was a one-time tournament that happened in March 2008.
Then there are the numerical mammoth ones - 10K kills, 1000 matches played, 20 ranked wins in a row, etc. Those suck ass.
Some achievements are just brutal, but obtainable - I have Mile High Club in COD4, 7 Day Survivor in the original Dead Rising - but there are some that are simply masochistic:
beat MEGA MAN 10 without taking damage, finishing all races without hitting ANYTHING in Ridge Racer 6, complete Smash TV without continuing
There are also a couple of games which have INSANE timesinks to get all the achievements - Rumble Roses XX takes about 200+ hours of grinding, and is notorious for being awful...but the game that takes the cake, in my book?
Final Fantasy XI.
You have to get to level 75 with EACH CLASS. This seriously takes
years.
I haven't really
been that bad lately, by the way...