Let's face it - Starbucks serves terrible coffee (especially when you are a black coffee drinker) but it does offer convenience and occasionally a nice environment to sit in (sans students studying and taking up the entire real estate). You should already know that the Citi Rebate benefit has been discontinued yesterday (25 April 2019) and you have only until 03 May 2019 to utilise any outstanding balance.
You used to be able to get 10% Citi Rebate from Starbucks and that is a fantastic deal since the Starbucks Rewards program (which comes with new benefits for Starbucks Rewards Gold members now!) already allows you to get 1 free drink with every S$60 spent. Now that Citi Rebate has been discontinued, the best way to spend money (including topping-up the balance in your Starbucks Card) at Starbucks is to probably first consider any on-going promotions that are running - Standard Chartered is currently offering S$3 off at Starbucks on Mondays with a minimum of S$6 spend but I do not believe you can use it to top-up the balance in your Starbucks Card.
On an on-going basis, getting 4 miles per dollar on your UOB Preferred Platinum Visa or 3 miles per dollar on the brand new KrisFlyer UOB Credit Card will probably be your best bet if no other promotions are running. Alternatively, if you have an American Express Singapore Airlines Credit Card, remember that you will get 2 bonus KrisFlyer miles on top of the base earn rate for the first S$200 spent each calendar month across participating merchants (yes, Starbucks is one of them!). It is definitely worth noting that you can get up to 7.3 miles per dollar at Starbucks with an eligible American Express Singapore Airlines Credit Card if you have the mobile wallet offer saved.
Get 7,500 more KrisFlyer Miles here: American Express Singapore Airlines Ascend Credit Card