check out PES tuning they offer the T28 but I think offer GT turbos also.

 

Since you have a B6, I'll answer with my opinion in a different way because your question could be split in to two parts, "best" and "most hp".
Best is the APR Stage III+. Very reliable. 340hp and 332lb-ft on a 1.8t. Very well thought out, good programming, and highest quality parts.
Most hp is toss up between APR and a tweaked GTRS kit with REVO and SPS3. Go to turbo section at http://www.eurocodetuning.comand see their GTRS and GT71R "complete solutions".
Note the price of all. You get what you pay for, before you question why one is more expensive.
If you want to plug and play, go APR. If you want to tweak, log, tweak, log, tweak, and then do it again when the weather or elevation changes (when you road trip or move), then go REVO/GTRS/GT71R.
Fastest low 12 second 1/4 mile B6 1.8t car is an Avant with the REVO big turbo option.