Oh yes, I love Spiritomb. I used one back when Diamond and Pearl first came out and that thing wrecked so many teams. Of course that's a couple generations ago already, and with new Pokémon, Fairy-types, and an entirely different metagame, Spiritomb has definitely seen some changes in usage.
I'm going to assume that this is meant for Singles, so going by that.
If you want a defensive Spiritomb, meant for walling and some stalling, I recommend a set similar to this:
Spiritomb @ Leftovers
Bold Nature
Infiltrator
EVs: 252 HP / 252 Def / 4 SpDef
- Calm Mind
- Rest
- Dark Pulse
- Will-o-Wisp / Sleep Talk
This runs like the classic Crocune set, basically. Bold natures boosts Defense while you'll be boosting your Special Defense with Calm Mind.
Keep in mind that Guts Pokémon (Machamp, Heracross), Fire-types and Fairy-types give Spiritomb some serious grief, since Fairy-types are super effective and Fire-types are immune to Will-o-Wisp, and Guts users get an Attack boost from the Burn.
Now, for your offensive Spiritomb, it typically runs more physical with a moveset like this:
Spiritomb @ BlackGlasses/Leftovers
Adamant Nature
Infiltrator
EVs: 252 HP / 252 Atk / 4 Def
- Sucker Punch
- Foul Play
- Pursuit / Taunt / Pain Split
- Will-O-Wisp
Of those three move options, I personally recommend Pain Split as it works like a pseudo healing move in the best situation, which would especially be beneficial if you decide to use BlackGlasses instead of Leftovers for the power boost.