How do I choose a furnace airfilter?
Depends a bit on your furnace.  If you have someone that services your furnace, ask them.

I used to use a pleated filter with a high MERV value.  The second time my furnace broke when it was well below 0 degrees, I spent some time talking with the service man.  Apparently the type of furnace I have does not draw air through these filters as well as it should.  This causes this part to burn out.

I now use the filters that are fiberglass screens and no longer have the problem.