In need of some recommendations of good FFT library

I want to perform Harmonic analysis on the main supply using FFT (taking 64 samples at 640Hz = 10Hz freq resolution) up to the 5th harmonic frequency and use it to compute parameters such as Vrms, Irms etc .... Does anyone know of a very FFT efficient library that can be performed quickly enough by an Arduino Yun?

Any of them should be fine. However, you should reconsider your sample rate.

As it is, any signal present at the input with frequency higher than 320 Hz will cause aliasing artifacts. You will have considerable difficulty coming up with a low pass filter than can reduce the unwanted frequencies and not affect the frequency of interest.