Very fond of Bundaberg. Still, as it's related, the best I've had is home made root beer. I have a home brewing/kegging setup, so the recipe is as follows:
1/2 packet of
Gnome Root Beer Extract2 kilos cheap white sugar
500g golden syrup
Dissolve the ingredients in hot water, then make up into 19 litre keg. Chill in fridge then carbonate.
This stuff just gets demolished at our place. It has a caramel-y, biscuit-y flavour. As it is poured from a tap, it gets that big old timey head on it. I've always meant to try other root beer extracts, but I just can't go past this one. It sells out fast from the link above.