I stayed at both the Hilton and the Conrad in SIN this past January. Though the Hilton wins points for location (and having quite possibly the most upmarket shopping mall in the world connected to its lobby), the Conrad wins hands down in terms of comfort of room, service, amenities, and breakfast buffet. The health club is very good and they have a nice pool. The staff are amazing and are just as good (and in some cases even better) than any Four Seasons/Ritz-Carlton property I've stayed at.
As mentioned, taxis are cheap in SIN and it's only about a 5 minute taxi ride to a central location such as th Raffles.
I'd stay away from the Mandarin Oriental. Though I didn't stay there, I did an extensive walking tour of the property. The lobby looks pretty dumpy and the immediate area that surrounds it is noisy and crowded. Service at their all-day restaurant, Cafe des Artistes, was subpar. It's a shame that this property, as well as the Mandarin Oriental Macau, is associated with the gem on Hong Kong Island.
If you want to splurge a bit, go for the Ritz-Carlton (outstanding views). They offer some very good rates starting at around S$260.
Otherwise, you won't be disappointed by the Conrad.