The wind turbine is, first and foremost, an investment, so a good wind turbine will pay for itself in a reasonable time and will start generating income (both financial and environmental) after that.
In order for a wind turbine to be a good investment, it has to have a good price to performance ratio. A wind turbine with lower performance and with a lower price may have a better ratio than a more expensive turbine with higher performance (and vice versa).
So, the first thing to look for is an annual energy production data (AEP), which shows how much energy does a turbine produce in a year at different average wind speeds. Also, the manufacturer or seller should calculate the return on investment parameter (ROI) for You. A typical ROI would be between 5-10 years, depending on the spot.
Another important parameter is power rating. Depending on the laws in Your country and Your energy consumption, You should choose the right turbine size for You. A good seller or manufacturer will help You with that.
You should also look for noise data. A good turbine will have it measured at 60 m and anything below 50 dB is a good result.
The aesthetic value is also important. Different people have different tastes, but if You want to see how a small wind turbine will look at Your spot, You can download our overlay and place it on Your photo with a graphic editor.
And the last but not least, a good small wind turbine will be certified according to MCS, IEC61400 and/or Your local standards. This will guarantee that it is safe, field tested, and that the data above (AEP and noise) is verified by a third party.
Once You have found a good turbine that You like, it is time to establish if You have a good spot for it.