IMHO out of those three airlines, Southwest is the better airline hands down. As far as Spirit and Frontier, I have heard horror stories of people being nickel and dimed with fees, bad service, etc..
More so than the three airlines above, I would recommend Delta which is the legacy carrier that provides service between MCO and Atlanta and provides much better service and larger aircraft, they also have flights nearly every hour between MCO and ATL which is their hub.
JetBlue will also start new service between MCO and ATL starting March 2018, that might be worth checking as well, they may have some introductory fares.