Best cashback/miles cards for grab top-ups AFTER UOB One has been maximized?

Amex

VISA cards such as UOB PRVI

UOB One and AMEX True Cashback.

There used to be more, but many of the credit cards T&Cs have since changed and you can now spot specific exclusions for Grab where it wasn't there before.

For now and still unchanged (till Dec 25th, 2019), UOB One (up to 10% rebate) and AMEX (1.5% rebate) offering no minimum spending for Grabpay top-up, other credit card providers just to shady as their T&C changes over the time and some could be effective after a few hours to a few days.

I had to set up website monitoring on the Banks's T&C webpages to get to know the latest changes from each bank, it's a hassle to set up at first, but it's a one time effort and provide me the latest updates on daily basis.

Be a wiser consumer nowadays is getting tougher and tougher as the Financial Institutes learning from the same or more platform like we do and closes the gaps as they couldn't reap profit or losing revenue.​​​