Your assumption translates into "not a Permanent Resident," so let's assume that.
If your spouse could hypothetically return to his/her country, re-establish residence there, and enjoy reasonable or better medical care in that country's public medical system, with minimum fuss, then a Singapore local Integrated Shield plan could be a good fit. Spouses from many developed countries, particularly from Europe, would fit that profile. Or if your spouse is more likely than not to become a PR at some point, given enough time, then starting now with an Integrated Shield plan could make a lot of sense. However, only select Integrated Shield plans are available to foreigners, at least to "dependent" foreigners such as spouses. Prudential's PRUshield Plus, designed for public hospital A ward coverage, is one such plan, and in my view it's currently the best in its class.
NTUC Income appears to be the only carrier offering an Integrated Shield plan geared toward public hospital B1 ward coverage (Enhanced IncomeShield Basic) to foreigners. NTUC Income is also the only carrier offering an Integrated Shield plan (Enhanced IncomeShield C) geared toward public hospital B2+ ward coverage to anybody, and they do sell it to foreigners. So those are foreigners' only Integrated Shield options below public hospital A ward level. The premium gap won't be as big, though, because foreigners do not benefit as much from subsidies, and consequently Prudential might offer the best value to foreigners among the Integrated Shield plan providers. However, NTUC's "C" plan is particularly interesting if your spouse has a pre-existing condition since it's better coordinated with the ward class (B2+ or lower) that he/she would most likely check into for cost reasons. If your spouse is female, please note that KK Hospital offers B2+ ward class, which is most probably the best value hospital ward in Singapore. (It's air conditioned! But it's also KK Hospital, and they're good.)
As far as I know you can pay for your foreign spouse's base Integrated Shield plan using Medisave funds, and that's helpful because those funds are tax advantaged. Other types of medical insurance are not Medisave payable.
If you can get your spouse covered through employer-provided medical insurance of some kind, likely in combination with Integrated Shield coverage, that's great. Or if your spouse him/herself is working in Singapore and getting some employer-provided medical coverage, awesome. Employers will sometimes do something reasonably sensible and coordinate their benefits with Integrated Shield coverage.
Now we're referring to "expatriate" oriented global medical insurance, and that can be the better option particularly when your spouse doesn't hail from a country offering a minimum fuss medical "backstop" and/or when your spouse is not expecting to stay in Singapore (not likely to be a PR in due course and to stay in Singapore in the future).
I had personal experience with group Cigna coverage for several years, and I loved it. It was gorgeous insurance, and I'm sure it was rather expensive, too. (I didn't pay for it, not directly anyway.) Claims processing was a bit on the slow side, and on a reimbursement basis almost always. But it was phenomenal coverage. There were no pre-existing condition limitations, no annual or lifetime caps, no waiting periods of any kind for newborns or for a new spouse, low out-of-pocket limits for covered services, and practically everything you can imagine and more was covered. It was also U.S. "Obamacare" compliant, and that's a good thing. Aetna, another U.S. carrier, is probably broadly similar for their top shelf group coverage.
If you're approaching your spouse's coverage from the non-group angle, then pre-existing conditions matter a lot. And the tricky part about pre-existing conditions is that an insurer won't cover them for individually issued policies (with some rare exceptions), so that means you'll naturally want to stick mostly or completely with the public medical system in Singapore. However, if your spouse is quite sure he/she doesn't have pre-existing medical conditions that could affect whether a claim is paid, then an individually issued "expat" policy could make some sense, particularly if your spouse's tastes in medical care run toward private medical care, if your spouse doesn't have reasonably fuss-free resumption of good public coverage in his/her home country, or if your spouse hops around the globe fairly often anyway. Decent or better "expat" medical insurance won't be inexpensive, though, but you can get some really good stuff if you're a good risk (no pre-existing conditions).
