What brand home ac dou you have? How old is it and what problems have you had if any?
Why do you ask? Are you in the market for a new one?
Armstrong 97% modulating gas furnace with a 16 SEER 3 ton 16 SEER. All manufactures out there make good quality stuff except Goodman. (Sorry Bigten) What needs to be researched is the cost of the parts when they start breaking down. This is where most of the big name manufactures (Lennox, Carrier, Bryant, Trane, Goodman) make all their money.
The other big thing to watch out for is to make sure that the contractor you are using knows his stuff. There are a lot of handyman type guys out there that think furnaces and A/Cs are just like replacing windows or doors, but in all reality its not even close.
If there are any other questions or ways I can help just ask! My pleasure to help my TOO brothers out.