UOB PRVI Miles is, on paper, the best general spending card in Singapore with 1.4/2.4 mpd on local/overseas spending respectively.
In addition to the good general spending rates, it offers 6 mpd on airtickets bought through Expedia with certain airlines. This promotion expired on 31 March 2017 but has since been renewed to 31 March 2018.
The catch is they’ve taken away the big daddy. Singapore Airlines bookings no longer get 6 mpd with Expedia (although, strangely, Silkair bookings do). This is definitely a big loss given that the majority of bookings people here will make will probably be with SQ.
With this change I’d say the best you can do now for SQ bookings is 4 mpd with your usual online spending cards eg HSBC Advance/ DBS WWMC. I know that from time to time DBS might do something with Expedia for 6 or even 8 mpd but I’m not aware of anything going on at the moment.