My greatest achievement has yet to come, I'm pretty sure. I don't have any kids yet, nor have I completed my education.
To date, I'd probably say mustering up the courage to go back to college at age 31. The social part wasn't so tough in community college (where the median age of the students is 32) but at the university, chock full of fresh-faced kids 10-15 years younger than me? Makes me feel about a thousand years old, but I'm not complaining about the eye candy!
If I can get through this successfully, graduation will probably seize the top spot on the accomplishments list.