All depends on what you want to do while you're here. A couple of thoughts:
1) The St Regis is excellent. Good service in my experiences, pretty good with plat upgrades, in Buckhead (tho night life there is second to midtown these days).
2) I'm not a huge fan of W hotels but if you are, there are three to choose from. One is in Buckhead, one downtown, one in Midtown. The Buckhead / Downtown W's are both pretty new, the one in Midtown is a renovated Sheraton. For the location, I don't know if you can beat the one in Midtown, even though its rooms may not be quite as fresh. I personally would not stay downtown, just because there's not much to do down that way.
3) I would avoid a few properties. All of the Sheratons are pretty bad and the Westin Peachtree Plaza is not good from what I've heard. The Westin in Buckhead, however, is not bad, tho it is worn.
There are obviously a bunch of SPG properties around town -- let me know if you have specific questions about some.
Hope this helps!