Best for audio workstation desktop? The most powerful and most expensive the $1000 core i7 980X.
Do you need it? Will it provide a significant performance benefit from a $100 processor? No.
You'll be fine with something like an Athlon IIx3, depending on what programs you use and how much you do at once. Generally this CPU is a safe bet and provides excellent performance at an affordable price.
In most circumstances there is not the "best CPU for a given task". They are all x86-64 these days and all work with all programs. All will do the tasks you need them to do. There are some exceptions such as if your programs use a lot more separate threads you would be better off with an athlon IIx4 over a phenom IIx2.
The real question you should ask is either:
What is the best processor for a digital audio workstation? I will be using xxxxx programs and my price limit is $xxx.xx. The rest of my system components are: xxxxxxxx.
Where the x's represent information that only you know.