Mine is tied with the Get them out if our Schools achievement from Splosion Man for killing ten Scientists and the To kill a Blackbird achievement for Shadow Complex, both at 99.3%. You would think everyone who has played these games would have these.

Bit of a stupid question, but how do you find out which is your most common achievement? I've been browsing through my GB achivements page and can't see any catagory that tells me what is my most common.

My most common is 'Fairy Godmother' which you get for purchasing Chime (99.4% - 7 people never got passed the start screen? Really?). Followed by Get Them Out of Our Schools and To Kill a Blackbird. 
 
Embarrassingly, my rarest is 'Party Animal' from Lips: Party Classics at 3.4% completion. I'll blame that one on the SO.

Get Them Out of Our Schools in 'Splosion Man and To Kill a Blackbird in Shadow Complex are tied at 99.3%.

I got to level 10 in World of Warcraft. I was totally expecting a Dorito's Dash of Destruction achievement to be the most common one. Then I realized that World of Warcraft counted.

To Kill a Blackbird followed by Freeflow Combo 5.

Get them out of our schools and to kill a blackbird as well.

Get them out of our schools & kill a blackbird... I see a trend developing here.
The rarest I have is Vendetta which is from DLC for Dragon Age: Origins (2.9%)

 According to Giant Bomb, my top 5 RAREST are:

1)  10 points
Ace 0
Have 8 cities in total Evolved game - Normal difficulty
Missile Command
July 25, 2009
    
2)  15 points
Fifteen Grand 0
Obtain a personal score of 15,000
Defender
May 23, 2007
    
3)  5 points
Special Delivery 1
Collect 80 hidden packages.
Hydro Thunder Hurricane
Aug. 3, 2010
    
4)  15 points
Demolition Man 36
Online: get 20 demolish kills
Battlefield: Bad Company 2
May 30, 2010
    
5)  20 points
Double Trouble 1
Win 2 Medals of each type (2x Point Buster, 2x Survivalist, 2x Time Spender)
Game Room
July 16, 2010


and my top 5 MOST COMMON are:

1)  15 points
To Kill a Blackbird 9
Destroy the helicopter attacking the vice president
Shadow Complex
Aug. 20, 2009

2)  10 points
Traversed World 2 0
Travel all the way across World 2.
Braid
March 1, 2009

3)  20 points
Secret Achievement 12
Continue playing to unlock this secret achievement.
Assassin's Creed
Nov. 17, 2007

4)  5 points
Catch the Fever 0
Cure your need for fever with an Extreme Fever. (Except Duel)
Peggle
May 10, 2009

5)  10 points
Vault 101 Citizenship Award 9
Got the Pip-Boy 3000
Fallout 3
Nov. 8, 2008

My most common is get them out of school my least common is Legolas in PVKII (0.3 have it)

" I got to level 10 in World of Warcraft. I was totally expecting a Dorito's Dash of Destruction achievement to be the most common one. Then I realized that World of Warcraft counted. "

I haven't hooked up my stuff to the site so I am not sure.

WoW: Level 10 (99.4%)

Fairy Godmother - Chime 99.4% 
Rarest is 100 Arcade Mode Wins in the first Marvel Ultimate Aliiance at a staggering 1.6%  I thought for sure my Vesperia Master (get all the other achievements) in Tales of Vesperia would be my rarest... but it's 3rd or 4th.

Freeflow Combo #5 from Batman Arkham Aslyum. 99.05% of people have this one. I wonder if there is an achievement everyone has...

My most common is for reaching Level 10 in wow(99.5%) narrowly beating out the achievement for buying Chime(99.4%) seriously how does that even make sense?
My rarest is for shooting down 10 enemies without dying as the biplane in Altitude(1.3%)

The flow of combat 1 is my most common, seriously 2.0 is my rarest.

Also the freeflow combo Batman one, followed by the next most common Batman one.  
 
It's interesting seeing which achievements people didn't bother to get at the very beginning of a game like that.
 
And rarest is apparently Platinum Rockstars in GH:WT :).  I played one of the two reasonably difficult instruments for that one so I'm pretty happy about it.

Most common:  
Secret assassin's creed achievement: 98,9% 
 
Rarest: 
Lips: Let's Get it STAR-ted  (40 points) with 0.7% :P

Paid in Fyrestone

Completed 5 missions in the Arid Badlands (  1,772/1,779 (99.6%) )

Get them out of our schools.

Completing the tutorial in Assassins creed.
98.9% got it.

Mine is a secret trophy from inFamous and for placing a sticker in LittleBigPlanet.
(my PS3 trophies still show from the two days we had PS3 trophy support).
 
My most common 360 achievement is for killing 10 scientists in 'Splosion Man.

Most common:
1. Level 10. 
2. Get Them Out of Our Schools.
3. Freeflow Combo 5.
4. Traversed World 2.
5. Secret Achievement.
 
Most rare: 
1. Cönquerer.
2. Sergeant Major.
3. Never Gonna Spend It All.
4. Sellout.
5. Subjügator.
 
I guess a lot of people play WoW and very few people played Brütal Legend online.