For non-singaporeans who wish to purchase hospitalisation coverage in Singapore, what's the best insurance plan? And if turn PR subsequently, is there any difference with the available choices?

Hi Anon, with relevant passes, foreigners can get an ISP for themselves and still keep coverage if they become PRs.

I would recommend taking a look at AIA Healthshield Gold Max A or NTUC Enhanced Incomeshield as 2 possible options for foreigners.