The major air conditioner manufacturers make multiple brands, and industry experts say that these brands are not distinct. (With other types of products, manufacturers often have a premium brand and a budget brand.) The logo on a unit or the appearance might be the only things that distinguish different brand products from the same manufacturer. Bryant/Carrier, American Standard/Trane, Lennox/Ducane and Rheem/Ruud are each "twin" brands. Goodman makes the Amana, Goodman, Janitrol and GmC brands.
Without the aid of credible reviews or guidance from contractors, we cannot identify the best models. As described above, the Goodman brands have detractors and defenders. They may be supported by contractors who are trying to sell on price alone. Contractor comments and inferences paint them as a budget alternative.
The models we mention here appear on the lists of Jim Dulley, GreenGuide.com and the few other sources that identify any specific models as worth considering. We emphasize that these are weak recommendations. They are not based on testing or even experience with a brand. Instead, they come from evaluation of product information and perhaps brand reputation. These sources focus on higher-end models.
The 20 SEER Bryant 187 Evolution Series stands out. It's a two-stage model, which means it operates on low speed most of the time. That keeps room temperatures consistent and minimizes electricity use. The Evolution Series uses R-410A (Puron) refrigerant. Bryant includes a ten-year warranty on the compressor and five years on other parts. The Trane XL 19i is a 19.5 SEER model that fits the same description, as does the 21 SEER Carrier Infinity Series.
The Lennox XC21 is a two-stage 20.5 SEER central air conditioner. It has a ten-year limited warranty on all "covered" parts. Lennox engineered this model with a variety of noise reduction technologies; the company claims noise levels as low as 69 decibels, which it says is "13 times quieter than a standard air conditioner."
The American Standard Allegiance 18 could be considered a step down from the above models. This 18.9 SEER air conditioner uses the outgoing R-22 refrigerant. It is a two-stage unit with a 10/5 warranty and is also made to run relatively quietly.
The 15 SEER Carrier Performance Series is another step down. It's also rated to run as quietly as 69 decibels. A 10/5 warranty is included. Some models in the series use R-410A, others use R-22. They are single-stage air conditioners (which means they are either on or off).
The Lennox Merit Series 13ACD is a 14 SEER air conditioner with a scroll compressor, which typically does not require a suction valve. It uses R-410A refrigerant. Those are nice features for a low-efficiency air conditioner. Because it lacks a ten-year warranty on the compressor, we identify it as a lower-end central air conditioner that might be a very good value if you are happy with the price or don't plan to stay in your home long enough to recoup the higher cost of a more efficient unit. A shorter warranty can be an indication of lower quality, but it can also be a marketing tool used to lower the sales price. A shorter warranty is especially worth considering if you don't expect to stay in your home for a long time.