Without having more details, generically I would say:
1.) CIMB FastSaver: this gives 1% with minimum 1k deposit. No fall below fee and no minimum amount. No hoops to jump through either. This will be markedly better than the (I'm guessing) 0.05% you're currently getting from POSB.
2.) Once you have 15k, and you're sure you won't be touching this money, can consider Citibank Maxigain as well. This one is more complicated in terms of T&Cs.
Maybe you can give us more information such as what your intention is for this amount of money, and how long it is before you see you have to use it (college education, marriage funds etc) and we can try and give more tailored suggestions. Generally speaking, if your timeframe is long enough (ten years and up), bank deposits is not a good idea as it is unlikely to beat inflation.