Three numbers describe glazing performance. U-factor is the rate of heat transfer through the assembly, where lower is better. Solar Heat Gain Coefficient is the fraction of solar energy admitted, where the ideal value depends entirely on climate and orientation. Visible Transmittance is how much daylight passes through.
There is no universally best combination. A north elevation in a heating-dominated climate benefits from higher solar gain. A west-facing wall of glass in a cooling-dominated climate needs the opposite. Specifying a single glass package for an entire house ignores the building's own geometry.
The largest real-world gains come from the worst starting conditions. Moving a single-pane sash with failed weatherstripping to a Low-E argon insulated unit with rebuilt seals is a dramatic change. Moving a competent double-glazed unit to a marginally better one is not, whatever the payback calculator claims.
Be sceptical of whole-home savings figures quoted without a baseline. Windows are one component of an envelope that also includes attic insulation, air sealing, and mechanical systems — and in most homes they are not the largest heat loss path.