There are quite a few 5 star hotels in SIN, so it depends but you are looking for, going through SIN 3-4 times a year for the last 15+ years, here's what I would say:

Raffles -- If this is your first trip, this is THE address in Singapore. It is actually a tourist attraction it's so famous. The rooms are quite large - high ceilings - fans, wood floors. It's Long Bar is famous and famously copied. Nice pool up top and small gym and spa. You also have access to the large gym and spa at The Raffles Plaza across the street

Shangri-la -- Sentosa is not the main one - the main one is called The Shangri-la Singapore. It is a "city resort" with a nice, lush pool area and also has a very good gym. Blu restaurant is good with nice city views.

The hotel has three wings and is quite big - The Valley Wing is positioned as the highest end with a private entrance and separate breakfast room.

Ritz-Carlton - not the traditional RC but quite modern. It's signature feature is windowed bathrooms with tub views looking out on to the city and harbour.

The Fullerton - Great architecture from the outside. The rooms are all quite different and while they have some nice duplex suites, there are some very small rooms as well. Also, the gym - unless it's been moved - is in the basement and quite dark - and small. The pool is nice, but small.

The Oriental Singapore - Mandarin Group - just re-opened about a year ago after a major renovation. I would classify it as more of a business hotel, however it is located right next to The Ritz-Carlton. *** There is another hotel in SIN called The Marina Mandarin, which is part of the Meritus Group and is near to The Oriental Singapore which is sister to The Hyde Park in London and The Mandarin in HKG - so often people get confused when refering to it - not sure if that's the case with the negative reviews above.

Depending on when you are going I find the SIN hotels are often running various promotions, so you might find upgraded accomodations - suites, etc. at decent rates. I always check both expedia, hotels.com and the hotel web sites if I'm price shopping.