Need cash when I land, best place in Brunei? Airport or places in the city?
Don't use the airport exchange! They have the worst rates. My advice is to use an ATM from a bank like HSBC in Bandar Seri Begawan. The rates are always better. You can use your card to withdraw Brunei dollars (BND) directly. Just make sure to notify your bank beforehand that you'll be in Brunei. The Brunei dollar is also interchangeable with the Singapore dollar (SGD), so you can also use SGD for cash.
The airport exchange counters have terrible rates. Your best bet is to find an ATM at a reputable bank branch in the city. The currency is the Brunei dollar (BND), which is interchangeable with the Singapore dollar (SGD) at par, so if you have SGD, you don't need to exchange at all. For getting cash from a different currency, an ATM from a bank like the Bank of China, Brunei Branch will be your best choice.
I've found that ATMs are the most convenient and cost-effective way to get cash. The airport exchange counters have awful rates. My advice is to use a Visa or Mastercard to get cash from a local bank ATM in Bandar Seri Begawan. The rates are much closer to the official interbank rate. You might want to grab a little bit of money from a counter just for a taxi if you don't have Singapore dollars, but don't do a large exchange.
Don't even bother with the airport exchange counters, they're always a rip-off with terrible rates. The best thing to do is find an ATM in the city. The Brunei dollar (BND) is tied to the Singapore dollar, so if you have Singapore currency, you can use it in Brunei. For getting cash from your home currency, an ATM at a bank like Baiduri Bank or Bank Islam Brunei Darussalam (BIBD) will give you a much better rate than any currency exchange.