It depends on how much work you want to do.
ffmpeg works pretty well for me and its free
It's used on linux (it looks like there is a windows version but I haven't used it.)
It works from terminal command line
Tweaking the outputs can be daunting (-sameq will usually give you great conversions)
Distortion could mean you are using the wrong settings OR
it could mean the program is unable to convert properly OR
it could mean your computer isn't capable of processing the conversion required by the program