I assume you're talking about lots of different currencies in relatively small amounts of each on a regular basis ? Unfortunately it's difficult to give a blanket answer.
If you go through Singapore regularly, the money changer at the Mustafa Centre gives good rates and is familiar with less common currencies.
There is a new internet based service called WeSwap which could be suitable.
firstratefx are okay for larger amounts.
Avoid the forex counters at Bangkok Airport, the rates are fixed identically and you will do much better in the city.
Avoid the backstreet changes in Bali no matter how good the rates seem, the little s***s practice sleight of hand.