Many music players that come with Linux distros can do it and you can even
use VLC. Best? That means different things to different people. Some
commandline junkies would say one thing and GUI lovers say another. A KDE
user would prefer one application while a GNOME or Unity user another. There
is SoundConverter (GNOME) and SoundKonverter (KDE) for example. either will
install in Ubuntu and both are in the repositories. You did not look very
hard. :) Tip: You can search Software Centre or Synaptic for mp3 wav and it
will give you suggested software.

Commandline love:
linuxpoison.blogspot.com/.../...rt-wav-to-mp3.html