Level 30 Super planet.
Not just super because of its size, but also because of its bonuses and its layout. I was able to terraform it upto a PQ39.
According to Brad there is no upper limit, only the physical limit of the number of tiles on a planet.
The max Planet Quality is generated by an inverse exponential function. Therefor the higher the class the less likely it will be generated.
However maximum number usable tiles is also limited by the terraformable tiles, and those are determined by the continent layout. The planet surface is generated using multiple 'randomly' selected 'stamps', which are handcrafted by SD.
As a result this means that the actual Planet Quality is the lesser of the terraformable tiles as determined by the stamps and the number of tiles as determined by the planet quality function. To get really high quality planets you must be lucky the function generates a high number and that the stamp layout will result in a high usable tile count.
Planet on the Galaxy map
And its planet view.