Sometimes brand can be important, but do not expect significant differences between the products offered by top AC manufacturers. The contractor and the installation are often much more important for AC performance than the brand…
Window-Room air conditioner brands
Frigidaire, LG or Kenmore models are at the top of customer and expert reviews. You may take a look at them in Amazon.com, here: Room Window AC. Other top rated brands: Haier, Friedrich, Rheem, Lennox, GE, Sharp...
For a wide list of qualified window-room AC Energy Star products, see: Energy Star Room AC
Mini-split air conditioner brands
portable AC brands
Contractors and AC service professionals
When selecting AC professionals, consider members of organizations like the SMACNA (Sheet Metal and Air Conditioning Contractors' National Association) or ACCA (Air Conditioning Contractors of America).
In Europe consider the AREA and its several national members: see AREA national members.
In Australia consider the AMCA and its associates.
The ACEE (American Counsel for an Energy Efficient Economy) has a guide on how to choose a contractor. See it here. See also ACCA, A technical guide to rate the quality of the installation of heating and cooling systems.