Utown outside dun need at above Starbuck, but no aircon, if aircon mean foodcourt
Usually I find best quiet place so far without pass is NUS-ISS , core of excellent, central library not bad, but need pass.
School of Computing sometime student discuss outside lecture hall will be noisy.
I only explored utown, central library , soc and nus-iss. There might other will be better like information system or techno building top floor, also nearby central library got those middle east or south asia school related might have some good spot.
Also the nus botanical garden the nus law there also have some aircon study area, maybe also a good spot