Here are some more differences between these four banks' zero minimum balance accounts:
1. DBS/POSB provides a full range of popular local services such as access to Singapore Government Securities (including Singapore Savings Bonds) and Supplementary Retirement Scheme (SRS) accounts. Consequently their zero minimum balance "My Account" may be attractive if you might need a full range of services.
2. CIMB is the only bank among these four offering a zero minimum balance current account (CIMB StarSaver). That means you'll get free paper checks with that account. Occasionally paper checks are still useful, although in the coming years they're likely to be phased out in Singapore. There's a minimum amount required to open a StarSaver account, but after that there's no minimum.
3. RHB and CIMB do not currently provide PayNow services and cannot currently be linked to Singapore CDP accounts (to receive cash dividends and interest).
4. There are more merchant promotions connected to DBS/POSB (#1 ranking in this dimension) and HSBC (#2 ranking) than with the other two banks. So if you care about getting 10% off your restaurant bill (for example), then having a DBS/POSB and/or HSBC debit card can be useful.
5. The debit cards that DBS/POSB and HSBC issue are compatible with at least some mobile payment systems, such as Apple Pay. RHB and CIMB are more limited in this way.
6. DBS/POSB offers a nice free PAssion debit Mastercard which is useful for accumulating Tap for More points, for example at Dairy Farm-related stores (e.g. Giant and Cold Storage supermarkets).
7. As far as I know DBS/POSB is the only bank among these four that issues a free debit UnionPay card. A UnionPay card might be useful in mainland China, in particular. (It's not a great card elsewhere.) You can have more than one free debit card linked to your DBS/POSB "My Account."
8. I believe CIMB (and possibly also RHB) won't facilitate transactions in/with anybody in Israel.
9. In my limited experience RHB is the only bank among these four that is openly hostile to U.S. persons. If you move to or otherwise have any U.S. indicia -- you go attend a U.S. university, for example -- then it's at least possible if not likely RHB will close your account or refuse to open it in the first place. Not cool, RHB.
I expect we'll see both the Singtel-Grab and Shopee digital banks offering zero minimum balance accounts starting in 2022. However, for now, there's already a great range of choices in this particular segment of Singapore's banking market. It's nice to see the improvements, and let's hope they continue.