I live in Atlanta (well the suburbs NW of the city) so I'll admit I've not stayed at any of the hotels other than the Whitley (ex-Ritz, now Luxury Collection, in Buckhead - really nice hard product and great service/elite treatment). Will you be having all your lunches and dinners at the conference? If yes, I've heard great things about the Glenn, and the rooftop bar is very popular with locals. If no, some of the best restaurants in the city are in midtown and midtown is quite walkable.

Please note, that MARTA does not run very frequently right now - not even during rush hour, and Atlanta traffic is horrible, so you probably want to avoid Buckhead unless your conference is in Buckhead. DUring rush hour it can take 1/2 hour to make the short drive to downtown (Buckhead is part of the city, fyi).