There is NEVER a 'best attack' in the Frontier. I, personally, am fond of Shadow Ball on Gengar, due to its added effect of lowering the opponent's Special Defense one level, and it dishes out mediocre damage, allowing me to strike back with Psychics, Thunderbolts, and Ice Punches.
Another attack I like is Magnitude. Sure, it may fail me at times with a Magnitude of 4, 5 or 6, but the PP usage is outstanding [30!!], the highest power being 150, higher than that of Earthquake, has the same effect [attacks all three Pokemon, except for some circumstances], and is deadly if against two Magneton or something.
My final favorite attack is Surf. It attacks both opponents - has an attack power of 95, and has a decent PP amount - 15. It's great for all Water-types with a high Special Attack, like Omastar and Lapras.