What is a good uPVC window or wooden double glazed replacement window solution?
Windows are very religious and debate inspiring. Identifying top goals will help in selection and maximizing ROI.

FWIW: uPVC and vinyl are synonyms in North America.

Energy Efficiency


Maximizing u-factor depends on the heating/cooling zone you are in.  .35 might be good for a northern climate, while overkill for a southern one
SHGC (Solar Heat Gain Coefficient) is more important in a southern area.
Air Leakage
Condensation reisitance


Intangibles


Durablity factors are hard to estimate accurately, but price is usually some indication of quality
Fit and finish  Construction materials used have an effect on
appearance as well as durability.  Maintenance requirements and paint-ability  are factors
Resale issues