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:

GPS US
QZSSJapan
BEIDOUChina
GALILEO EU
GLONASS Russia
IRNSSIndia

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:
https://www.gnssplanning.com/#/settings
Use this settings page to set your location, click Apply, then click world view tab.  Note even the Japanese QZSS flies over Australia.