There's a ton of discussions on that over at the Singapore destination forum, but in general, it depends on what you're looking for, budget, etc...

Reasonably priced, business friendly hotel, I preferred (before moving full time) the Quincy. Its tucked back a bit from Orchard, so not the easiest walk to the MRT when super hot, but includes free meals and happy hour, as well as free laundry and internet, with a great staff.

Luxury, I would recommend the Fullerton Bay, the Mandarin Orchard, or the Shangri-la (though not the one on Sentosa, unless you plan to mainly hang out in Sentosa).

For local fare, any of the hotels around Bugis or Chinatown will do you well.

Again, I'd hit up the Singapore forum on Flyertalk for much more, as the hotels in Singapore and best locations is a hotly debated topic.