Some of my good pictures of the Singapore skyline were taken from :
1. Benjamin shears bridge (take the steps in front of Millenia Tower and walk along the bridge till you come to a spot from where you think is the best view). BTW, this is also a good spot to take pictures of the fireworks during CNY etc. Its jampacked during NDP - so forget about going that night.
2. Marina waterfront (the jogging track in front of Ritz, Marina Square etc. i.e. next to Esplanade)
4. Boat Quay (next to the new parliament building)
If you want to take traffic trails or night scene, I think it is best around city area
If you want to take star trails or nature night scene, I think you can try Punggol or those ulu places where there is lesser light pollution.