What's the most cost effective way to fund my TD Ameritrade account?

Pretty much what Jonathan has mentioned. So usually if I have no urgent need to buy a stock, I will just use the DBS transfer to TD. It'll take up to 2-3 days for this. If there was an urgent need, you can instead wire USD directly then. You can get the amount by the opening of the market if you transfer in around noon.

Hi there. If you have a DBS / POSB account, you can transfer SGD to TD Ameritrade Singapore's DBS account and they will convert to USD for you. The rate is not published but it's pretty close to spot. This process takes about 2-3 working days.

Alternatively, if you have a multi-currency account, you can use that to convert your SGD to USD at a rate you think is favourable and then wire the USD directly to your TD account in the US. For this, it's pretty fast. I can wire in the afternoon and the funds will be reflected in my account around midnight or so.

I did a comparison between transferwise and DBS' published fx rates. Seems that DBS still wins for some reason after accounting for transferwise's fees.