Ground type on the type coverage checker lies about its coverage and everyone has an answer for EQ!
The best 4 moves for most supereffective coverage I've found is:
-Any Rock attack
-Any Fire attack
This combo hits at least 550 Pokemon supereffectively (type coverage says 529 because it does not count Pokemon hit supereffectively by the Flying part from Flying Press and Water types hit supereffectively by Freeze Dry), but since it is Smeargle using them they'll do terrible damage anyway.
If for coverage, then AWildNoobAppeared nailed it. If for sheer damage, then that would be...
The scary thing is Rayquaza can have the three latter moves...
Hope this helps!