Hmmm...good question....
A lot of my greatest achievements are looked at in retrospect, in other words I didn't appreciate them at the time.  Like getting my eagle scout or getting accepted into a very competitive study abroad program.  They just seemed natural and kind of cool at the time, but nothing more.
I guess I would say my greatest achievement is my girlfriend of 3 years, she great.
My greatest achievement not including her is getting an A on my first Chemistry test this year, my grade on that test was going to judge whether or not I am throwing away 2 years of my life, and I dominated it.
Also, my junior year in college, when I was in Chile, I got so drunk one night I got lost in Valparaiso.  I ended up in a sketchy ghetto with guys throwing cans at me and threatening me.  i don't know how I got home or what happened, but I woke up in my bed safe and sound.  that's all i remember