What GNSS systems to enable?

With a choice of satellite systems from the different countries, which one shall I enable in my device, for better performance such as time to first fix (TTFF), and battery usage?

GNSS Constellations:


The more satellites enabled do not help that much in better TTFF, but it does help is difficult environments where there are obstructions such as device casing and/or tall buildings, limited parts of the sky are visible.  Generally GPS and GLONASS are the most established and give the best coverage.  As a general rule, use as many constellations as you can for better reliability/accuracy, with priority to GPS + GLONASS.

If you are looking for better overall fix/power consumption compromise, then you can consider only using GPS + GLONASS.

A particular constellation may have better coverage depending on your country/region. Australia is one of the few countries where many different constellations are visible.  To see the actual near real-time trace and satellite paths:
Use this settings page to set your location, click Apply, then click world view tab.  Note even the Japanese QZSS flies over Australia.