Personally I would have Claydol's moveset be:

Cosmic Power
Psychic
Earthquake
Toxic

For the 3rd Gen games, this movesettargets all of the Pokemon types some how and Toxic is mainly used to weaken usually hard to defeat Pokemon like Altaria found within the game. Any status inflicting move will do nicely. If you transfer Claydol to the fifth Gen, then I would agree with the others in saying that Zen HeAadbutt would be a good 4th attack option. Earthquake has always been one of my favorite moves because it hits hard and Claydol would get a STAB from it.