It has been much discussion around the power supply voltage of the transformer for Arduino, but nowhere is it discussed the current power supply must provide this.
So I would please ask in this topic to summarize the desirable characteristics of an external power supply (transformer) ideal for powering our arduino one, thanks.
I feel that this is a bit of a general question that can be answered in a million ways.
The only good answer imho is : check the specs of the arduino in question and adjust the power source to your needs.
If the power source is within specs and the form factor suits your needs; your in business.
BTW a transformer alone is not enough, you will need a diode bridge, voltage controller and some capacitors...
Just my two cents 
Why you need a bridge of diodes, capacitors, voltage regulator and some ... ?
You are mentioning a transformer and that component typicaly supplies a AC voltage.
The arduino need DC voltage to function.
I suggest you look into the basics of this before you use you arduino, if you get it wrong, you can fry it.
BTW, there are a lot of power supplies out there that can connect directly to an arduino without any alteration.