We use Meadowbrook Air Conditioning and Heating. They are a small company, however they are very reasonable on their prices. They don't have a huge overhead and a building where other big companies do that overhead and pass that cost of upkeep, employees on to you.
We have an older home and we were on wall and window units for quite awhile. We finally decided to get central heat and air, however it was expensive no matter who we used. However with Meadowbrook AC it was considerably cheaper and they were reliable and fair. The owner my husband got a referral from at work from his boss.
We've had our system a year now and we've had no problems out of it. We asked them to install a light in the attic and they did that for us too.
I would highly recommend them. C.