I've used a few brands over the years and have to say they all performed well. The two features I'd be concerned with are the energy consumption rating and the sound level. Both should be prominently shown on the spec tag.
I assume you're asking about wall/window units.
EDIT: To add....I'd probably use the same thinking when choosing an AC brand as I would for any product. Stick with the well known quality names. IMO, Friedrich, GE, Carrier etc are a safer bet than Sanyo, Sankyo, Sharp ect.
I've never use a Japanese model like Panasonic or hitachi but I'll bet they are well made.
Great place to research:
http://www.epinions.com/Air_Conditioners