'The best' is always a totally subjective and unproveable concept.
Having said that, I always use 'soundconverter'. It is in the repos and has a gui. You can install with
$sudo apt-get install soundconverter
at a command prompt (terminal) or search for it in Synaptic Package Manager.
Alternatively, you can use "File --> Export" in Audacity. I am pretty sure they use different codecs so you might want to try each method & see if you have a preference for the resultant quality.