There is no consensus for the "best" nature for a Spiritomb. It's a wall by all means, so the nature should generally benefit one of its Defenses, as the EVs should go toward maxing out HP, and dividing the rest between both Defense and Special Defense.
I use Sassy myself, since Spiritomb ain't gonna be outrunning anyone important this millennium. But it could also depend on your moveset.
Impish could work for a Walling TauntPuncher.
- Taunt
- Sucker Punch
- Pain Split (Egg Move, sadly)
- Will-O-Wisp/Hypnosis
What you do is Taunt an enemy who you predict will try to Dragon Dance or Swords Dance up, or even Nasty Plot, up and use Sucker Punch for a cheap hit. Pain Split is a pseudo-recovery move, and the last slot is filler. I wouldn't suggest Hypnosis on the TauntPuncher, because when your enemy can't attack, neither can you.
Dark Pulse is a better option on Spiritomb. Here's a layout of my Spiritomb that works amazingly well (just ask people who battle me how utterly annoying it is):
Spiritomb @ Leftovers
Sassy/Relaxed Nature
EVs: 252 HP, 129 Def, 129 SpDef
- Dark Pulse
- Paint Split
- Hypnosis
- Will-O-Wisp
Now, you may be thinking, "Why go with a -Speed nature when you're not even using Attack?", and the answer is simple: With one of those natures (depending on which Defense you choose to boost a bit more), you can safely switch between Dark Pulse and Taunt/Sucker Punch without worrying about a decrease in offensive power. Simply replace Dark Pulse with Sucker Punch, and Will-O-Wisp with Taunt, and you can surprise someone you've battled before. And when you're done, you can switch back with a Heart Scale and an extra Will-O-Wisp TM, and vice verse.