I just answered a very similar question, so copy and paste for the win!!
Though Smeargle is kind of bad, he is capable of the best super-effective coverage possible.
With the moves Freeze-Dry, Flying Press, Earthquake, and Shadow Ball, Smeargle hits all but 78 Pokemon super-effectively. This is the best I have found so far.
It would be increased slightly if Smeargle could copy Thousand Arrows instead of Earthquake, but that is currently impossible, and anyway, it would only help against three or four Pokemon. (Charizard, Ho-Oh, Moltres, etc.)
I don't think one Pokemon can hit all Pokemon for super-effective damage. Maybe with a few more moves like freeze dry and flying press, this novelty Smeargle can some day achieve this goal.