As you might expect, we've done a ton of work on transformers.
The "best" transformer needs to be matched to the specific ribbon motor, ribbon type and there are other factors as well.
You can't just spec a brand without considering things such as efficiency, noise, frequency response (especially at the low end), hum aperture, and turns ratio. What is the source Z of the ribbon? That matters very much. Do you want a very low noise floor? If so, then you don't want a high ratio, which many have.
In general, a more efficient transformer will sound "better", and have lower noise. Size is an important factor, and bigger is more efficient as that usually means lower Ohmic loss because larger conductors can be used. There are also several types of transformers to choose from. Some EI types, toroids (not a favorite here), multi tap, etc.
Here is a link to some stuff about ribbon mic transformers.
Latest News from Crowley and Tripp's Lab: Search results for ribbon microphone transformers