Hands down, The Arcade at Raffles Place.
Walk in from the street, note the rates at the two booths on the left and right of the entrance. Keep in mind that they will likely not be the best of the bunch.
Take the escalator up walk the length of the floor and note down the rates at the various booths. Be aware that one place quotes it's prices as as Offer-Bid (!) rather than the usual Bid-Offer so read the boards carefully. I'd then pick the two best from the list, and go back and ask them what the best rate they can do is for S$5k, if you're lucky they'll add a pip or two and that might be maybe another $10
Only caveat with The Arcade is it is not open on the weekends (or it might be on Saturday morning). Plus try and avoid lunch hour if possible as the queues can get a bit long. The place I seem to keep going to is called Sheen Intnl Pte Ltd, 1st floor, about 2/3rds of the way down on the right.
I'd defy anyone to get better rates than can be found at The Arcade. As I've previously reported here, I once sold around S$5k for US$, and we got them at a price better
than the live money market rate (I had a Reuters FX pager in my hand!). Now that guy obviously had a position he really wanted to unwind and so was willing to take a small loss, but the spreads there are miniscule anyway, and it shows what is possible.
Also, I have never had any of them charge me a commission, everything is in the spread.
p.s. On weekends or when getting to The Arcade is not convenient, I go to Mustaffa (you can check their live rates on their www). Still better than any of these one man and his dog change kiosks. When dealing with a wedge of notes the change windows on the street are a bit of a zoo, so I'd suggest entering the shop and using the windows facing internally.