It really depends on your team, and where you are in the game. If you chose Typlosion or Samurott, it's probably better to choose Sneasler, as your team already has an edge over Grass, Psychic, and Ghost (Typhlosion) or Ground, Psychic, and Ghost (Samurott), and Sneasler takes care of Fairy, Dark, Normal, Ice, Rock and Steel.
If you chose Decidueye, though, you already have a Fighting type, and a Poison type isn't super useful. However, a Weavile works well to take out the pesky Ghost, Psychic, and Flying Pokemon.