What is the best command to simplify curves? Fair, FitCrv, Smooth, Rebuild, or RebuildCrvNonUniform?
the best is a vague definition at best, simpler in acceleration variations or simpler in matters of less cp? you might have to specify that a bit, i am sure each command serves a well thought through purpose (i hope 
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fair might even make the curve more complex if its not degree 3. fair is also not to simplify but to equalise accelerations in the curvature. smooth is not doing anything other similar to fair, i am not sure what the actual difference is but smooth also works on vertices and surface cp (i think)
fitcrv will only simplify the curve when you increase the commands tolerance to 10 or whatnot but then it could be actually a good choice since it more or less stays within the shape. if you have a lower tolerance it will make it more dens.
RebuildCrvNonUniform is one of my favourite, that also works very well if you set up curves for loft, it will rebuild the curves so that they match points to some degree.
i would say rebuild is good when you want the shape to be approximately there but with equidistant cp.
it happens that i use divide to place points along a geometry and use CurveThroughpt set to control point curve type, that will create very smooth geometry.
then you also have simplify but that only works on straight lines and something that in fact is very close to a circle converting it into a circle. i believe it does nothing to curvy nurbs.
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That’s a comically vague question from anyone who has ever actually used Rhino, I don’t know what the point of such a question could possibly be, but the “best” method is drawing a new curve by hand, the end.