Atlanta varies a lot with traffic. I come in from Athens, about 90 miles away. It's a 90 minute drive, I usually give myself three and a half hours before the flight. Mostly I get there really early, but beats missing a flight due to traffic. They move you through security pretty quick, even on Thanksgiving I think I got through in about 15 minutes. Never gotten searched there either, they'll let you through without being too bitchy almost all the time. Getting between terminals is easy, you can walk or take the underground tram. It's all linear too. It can be a bit of a walk once you get into the terminal, the low numbers gates are a LONG way from the center where you start. Transfers aren't so bad either, because of the terminal setups.
You can get to the airport on MARTA, but it's a bit of a limited system in terms of access. Never tried it, but I hear it works well. Traffic in Atlanta is hideously variable, I missed a flight one time because the idiots decided to close three lanes on I-85 the Tuesday before Thanksgiving for construction. Queue up two hour traffic nightmare.
DC: depends where you're coming from. I was always coming from other places in Northern VA, so mostly flew out of Dulles. It used to be awful, they had these stupid shuttle buses to get the terminals and they were not efficient at all. Now they have an underground tram, which has made things a lot better. Still not quite as good as Atlanta, but traffic is lower. Seems to be getting better overall.
I've only been through Reagan once. Sort of an annoying setup, old airport, but security went super-quick and there aren't all that many gates total. No real problems, it's just old and ugly looking. Reagan has a Metro stop, Dulles does not, so there's a big advantage if you're coming from the city. They're supposed to be adding a line out to Dulles, but God only knows when that'll happen.
La Guardia is sort of stupid for transfers, none of the terminals are connected. You need to go outside, then take a shuttle. Was very annoying. I'm not from the area so can't say much more than that.