There are some really nice neighborhoods in Atlanta for 20-30 somethings. The "best” neighborhood would depend on your own personal lifestyle. What types of things do you like to do? Are you single? Do you have a family? All of these things come in to play when you are picking out your neighborhood.
If you have a family and are looking for a quiet little place to live, you might want to consider neighborhoods like North Druid Hills, Dunwoody, Sandy Springs or Candler Park.
If you are single, and are looking for fun and vibrancy, there are a variety of locations that cater to different personalities. Buckhead is a nice option for trendy well-to-doers, who want to be close to fancy shops and elaborate nightlife. Midtown is a nice option; it’s perfect for free spirits looking to enjoy high rises and have a good time.
Then, there are those artsy historic little neighborhoods, that cater to the hip folks who appreciate vintage boutiques and exposed brick lofts. These neighborhoods include, Little Five Points, East Atlanta, Castleberry Hills, Poncey Highland, and Cabbagetown.
If you are a nature lover, you might enjoy neighborhoods like, Piedmont Park, Grant Park, or anyplace that is near the Chattahoochee River. I'd say first, find out what things are most important to you, and then look into neighborhoods that cater to your specific individual desires. That's the best way to do it :)
There are so many options to choose from, so don’t limit yourself. Have fun!