Singtel. However, be aware that most mobile broadband are using
3G for data transfer, and if the 3G antenna ain't present, your
speed will just drop to accomodate GSM. One of the biggest
blindspots have to be Outram station to Tiong Bahru.
Alot of HDB flats also do not have it, so you'll be getting
ranged signals from the nearest antenna, or not get it at all. The
higher you live up, high chances you'll get weak to no 3G
signal.
I live on the 17th floor, and I got no 3G, until I step into the
lift. Can be a pain sometimes when your home wifi doesn't reach far
enough to the corner of your home.