There is no such thing as 'which USB audio interface is best' for MMM. The unit another poster may recommend may not suit your needs.
It is a case of which USB audio interace is best for you ie has the facilities you want and are likely to need.
Make a list of what you need eg
Mic and Line inputs, Mixing capability, stereo/surround sound, low noise, minimum latency etc.
Once you have done this look for suitable units that fulfil your needs (or most of the important ones - two of which I have highlighted) and ask here if anyone has these units and their opinions.
Thanks, John. Most;y I'm using MX for a songwriting sketchpad. I just want to throw in a guitar track or vocal here and there. I want to keep things as simple as possible. I've noticed some units come with Pro Tools "M" or Cubase. I don't know which is best (or easier) to use. Demo quality is fine, but finished quality is not a must.