Given that the premiums aren't guaranteed I don't think you should take minor premium differences into consideration. They could vanish quickly.
Which do I recommend? "It depends." Oversimplifying only slightly, for citizens I suggest Great Eastern's SupremeHealth B PLUS as a reasonable baseline. If you have covered all genuine insurance necessities and want to splurge, you could buy something more expensive (and with more benefits), but that particular plan is a nice baseline -- currently best in its class (designed for public hospital B1 ward coverage).
The baseline (non-luxury) coverage for foreigners and PRs is probably a choice between Prudential's PRUshield Plus, Raffles Shield A, and AXA Shield Plan B. These three plans are bunched close together and very competitive with one another.
Foreigners probably ought to buy the lowest cost rider that caps their annual expenses for covered services unless they have employer-provided coverage that fills such gaps pretty well. For citizens and PRs, the lowest cost rider that caps annual expenses for covered services could have merit, but citizens and PRs with significant wealth, fat MediSave balances, and/or employer-provided coverage could probably skip the rider.
No, not really. There's ComCare, but you definitely won't enjoy living on ComCare as your sole or primary source of income even if you did qualify. (It's quite tough to qualify.) CareShield Life will be compulsory for younger cohorts sometime this year, but CSL is only for profound disability ("3 out of 6 ADLs") and also a very low amount of income (but more than ComCare). DII is really, really important stuff for most working people.