Best depends on what's important to you.
If you're talking about reception then they are all much of a muchness. You'd be better of just googling what phone has good reception (and even then you're looking at variations within the same model)
If signal strength is a critical metric then Telstra do have the best coverage. You will get much more variation in signal strength with different Telcos than different phones. Perhaps grab a few $2 prepaid SIMs (Vodaphone, Optus, Telstra) and see what works best in your area?
That said, Optus covers much the same area as Telstra (just a bit less in regional areas) so it may be fine (whatever the phone)
Hi there KenLeung,
Come back and let us know what you decided to go with ☺️