Would like to seek your advise on the ownership of my parents' HDB flat in the event when one of the owners passes away.

Yes, that's how joint tenancy works. Your parents own the whole flat together, and when one of them passes away it goes to the other automatically, will or no will. That's survivorship. So a will made by the first parent can't pass the flat to you. It goes to the surviving parent.
​

Tenancy-in-common is the other way to hold it. Each owner has a separate share, there's no survivorship, and each share follows that owner's will, or the Intestate Succession Act if there's no will. HDB explains the two here.

​

I came across a case that shows how this plays out. A father and one of his daughters were joint owners. A few months before he died, he made a will leaving his share of the flat to his son. The will couldn't move the title, and the flat went into the daughter's name. It didn't end there though. Her sister went to court for the estate, and the appeal court found the daughter was only holding the flat for it, because of what the father meant when he first added her name.

​

I'm learning how these rules work through real cases, and started a YouTube channel to share them. That case is here.

Yes, will is best and also to discuss with all parties to ensure there is no dispute. If there is a will, the will must be proved and executed under probate. The appointed trustee or executor will oversee the process following the Probate and Administration Act.