1) The Swap Offer Rate (SOR), or
2) The Singapore Interbank Offered Rate (SIBOR)
However, a banker might sometimes offer you a third option: an Internal Board Rate (IBR), often just called a "board rate". Using the IBR means your home loan interest rate will be determined by the bank.
And they can change it as they like, without you getting so much as a sympathetic nod and a "sorry".