AFAIK UOB One card has the best cash back however there is a minimum that you have to spend to qualify for it. And there are tiers as well. However, unlike Manhattan, the tiers for UOB One are not "joined" for the lack of a better word.
For e.g. the first tier is $300 to $500 and the next tier is $800 to $1000. So if you spend under $300 you get no cash back. If you spend more than $500 but less than $800, the cash back you get is based on the first tier.
Manhattan card has no minimum spend and the tiers are not "joined". For e.g. the first tier is $0 to $300 and the next tier is $301 to $500. So if you spend more than $300, your cash back will be based on the second tier not the first tier.
Personally, I would rather use Manhattan. My monthly spending fluctuates. There are times that I can't meet UOB One's minimum spending. Plus I hate that their tiers are not "joined". So to each his own but for me, Manhattan is the card to go.
BTW I'm talking about Manhattan World card. I know some lao jia here have other variants but mine is Manhattan World.