I think that the best moves for Swampert are:

Surf - Powerful because Swampert is a Water type too.
Earthquake - Very powerful attack, effective against electric and steel type.
Strength - Quite good, and works well on all types.
Ice Beam or Blizzard - If Swampert learns this move, then he will be no more vulnerable to grass or dragon type. Effective against flying type too.

Therefore, your Swampert will be undefeatable.