You will find a lot of differences in opinion in this one. Here's mine.
Trane and Lennox are very good. Lennox can be hard to get parts for depending on where you live. Bryant is cheap version of Carrier. Carrier is OK but not at same level as Trane/Lennox. The J in Janitrol stands for Junk.
I don't know much about Williamson or ThermoPride.
As far as AC goes, Carrier is a good money maker for service companies. And you get to decide whether Puron is the coolant of the future or not. I gamble I wouldn't take. Don't let Carrier scare you with the auto AC comparison.