Difference between par yield, forward yield and spot yield for bond prices?

Which one of the 3 yields is commonly used in calculations such as the benchmark for the risk free rate?

 

Classically, it used to be par yield. However, given the current state of affairs, it's not even clear that using treasuries is the correct approach.

 

What do you mean by that?

 

Here's a good summary:
http://www.pimco.co.uk/EN/Insights/Pages/Finding-Sea-Level-for-Interest-Rates.aspx