There is no "best nature" for any Pokemon. But for Pokemon that focus on physical moves (e.g. Gyarados), get a nature that decreases special attack, and for Pokemon that focus on special moves (e.g. Milotic) get a nature that decreases attack.
In Samurott's case its attack and special attack are nearly the same so you can do one of three things:
Make it a physical attacker and have Jolly or Adamant as its nature.
Make it a special attacker and have Modest or Timid as its nature.
Make it a mixed attacker and choose any nature, maybe one that decreases defense?