Aqua UV is the best by far, I have installed several hundred of these on ponds and tanks of ALL sizes (10-500,000 gallons). PM me and I can point you to the best price around for a new unit.
Emperor Aquatics comes in second, but but the housing becomes brittle after a few years and warping makes a good seal hard. The light quality is also not as good and more difficult to get replacements for.
Tetra, not worth the money. The bulb and the ballast don't put out near the micowatts and wavelengths they say.
I have dealt with over a dozen different companies UV's, Aqua is CLEARLY the best on all accounts.