For graduate admissions, are difficult classes valued over perfect grades?
I think this is a pretty easy one.


  I know looking at grades in isolation w.r.t. admissions is a questionable thing to do in the first place, but from an admissions perspective, are difficult classes valued over perfect grades?


Yes.


  I guess the example in mind would be, is it better to have a couple of difficult courses + mostly undergrad courses with all As, or a lot of difficult classes but some Bs sprinkled in?


The latter is distinctly better.  I got a B in the first graduate course I took as an undergraduate.  (I then took 8 more graduate courses and got A's.)  This did not stop me from getting into all the math PhD programs to which I applied, including the top three in the US.