Which air conditioner companies make the best new air conditioners?

That depends what you mean by "best".

The "best" answer I can give without any specifics is to look at the Freidrich line. They have everything from models with a little extra performance to explosion proof units rated for hazardous areas. Their better models have things like copper instead of aluminum tubes, the steel is thicker, etc. Some are made in USA, you don't see that in other brands. Don't bother looking for them at Wal-Mart!