Which reward points are worth more and has better redeemable rewards? $DBS, $CITI or $UOB?

I think Citi UOB DBS.

Not that the exchange rate is different, but for DBS points, the best card to earn them is DBS WW which earns 4 mpd. But the DBS points earned by DBS WW are only valid for a year, which is a fairly short duration. This means you'll need to make frequent conversions and incur more fees. And if you're earning DBS points via Altitude, while they last forever, they're earned at a rate if only 1.2 mpd, which isn't great.

Citi points either don't expire (Citi PM) or last for up to 5 years (Rewards), which is a long time to rack up points before converting to miles. And as someone mentioned, they have more transfer partners than any other bank.

UNI$ only last for 2 years regardless of which card you earn them with, but that's still longer than DBS points from DBS WW, which means you'll likely need to make only half as many conversions to miles and save on the conversion fee.

If you intend to change for miles, Citi points are considered most valuable as they have the most transfer partners