My approach is to do a quick scoot past the entrance and down the 1st floor (i.e. up the escalators), and note down the rates on offer.
I find a lot of them will have the same rate. The two by the street entrance are usually not quite as good, but I suppose they get the lazy passing traffic.
The one I use most is on the 1st floor, opposite POSB, on the right. It is called Sheen Intnl. Last time I used one also on the right but a little nearer towards the escalators. That one really is almost a cubby hole/glass fronted kiosk.
To the unitiated watch out for the guy (just after Sheen on the right) who quotes offer/bid rates, rather than the standard bid/offer, as that might confuse you. Also on the Ringit you can get some frankly confusing quotes... .42-2.4, i.e. half being quoted inverse! You might want to take a calculator.
p.s. When I said 'trading in size', trust me I don't mean size to buy a Bentley

. Just the kind of sum I expect your Joe Average Expat might need to do a few times a year. On such sums you can get them to shave their rates a touch.