The three at Lucky Plaza basement are pretty good. I suggest going there as soon as you can when you arrive since if you can't find what they have at the store, generally they can get some other items there in a day or two. Unlike many other things in Singapore which are now more expensive or as expensive as Thailand, scuba gear is still significantly cheaper. I bought a pair of Mares Quattro there for just under 3000 bahts, did not even bargain much, in comparison to 5000-6000 bahts here in Thailand.
They also have some wetsuits there.