Lame (used as the conversion core for many other apps, or can be used by itself via frontends like Lamedrop or from the command-line)
iTunes (you can set it to use MP3 or AAC from the Preferences menu under Importing)
I'd only seriously recommend one of those two. I generally use Lame from the CLI, but it's also integrated into things like Audacity (IIRC), ffmpeg (not by default, but it can be compiled in - hence any ffmpeg frontend like WinFF with an libmp3lame-enabled build of ffmpeg will work), old - and new, but crippled - versions of dBPowerAMP Music Converter, etc. iTunes would be fine if you don't really want to think about it.