Which are the BEST quality UPVC windows to install?

I have been in the window industry for over 30 years , rehau although good have cut corners over the years to compete with cheaper brands , reducing thickness of profiles where they can get away with it , but its not about the profile its about the way its manufactured and fitted , good quality hinges and locks, reinforced where necessary and the right screws in the right places, the measuring of the window plays a big part in how good the window looks and performs when fitted , must be fitted squarely with the right fixings and then sealed properly, I'm not a sales man but I could bore you for hours , I use Deceuninck and I don't get any problems I also fit a cheap window for a bigger company and once Ive sorted out the manufacturing issues they are ok

Kommerling hands down.

Rehau are top quality but not A rated on the 70mm system unless thermally sleaved. Ask if you are buying Rehau. Selecta 70 Advance look exactly the same as Rehau and come with an A rating. We are trying it out and we are happy with it.

I supply and install both Rehau and decerninck, both quality profiles and both good looking profiles. As regards the A rating Rehau can obtain this with the 62mm profile.

there are various window profiles on the market and the german profiles seem to be the best with regards to fewer leakages, better insulation properties, less brittle pvc, stronger re-inforcing and longevity. Profile names include Rehau, Kommerling, Schuco and Veka but in my opinion Rehau is the best profile with innovations that others seem to follow and if manufactured properly has much fewer remedial works

As already stated,REHAU are the windows that lead the way.

As above. But generally, it depends on the manufacturer. From a fitting point of view, I prefer Rehau over others.

Spectus are what most my customers go for out of kommerling they prefer the whiter look.