Thunder wave, definitely. Not only does it make the defending Pokemon have a 25% chance of not attacking, it also slows them down. I'll show you something:
Turn 1:
Foes Pokemon uses Surf
You use toxic.
Turn 2:
Foes Pokemon uses Surf
Your Pokemon fainted!
See, all it takes is a Pokemon with good Attack/Special Attack to 2-hit KO most
ghost-type Pokemon, which usually have low hp. You wont even get to use Hex! Now take a look at this:
Turn 1:
Foes Pokemon uses Surf
You use Thunder wave
Turn 2:
You use Hex.
See, a nice 100 power (or 150 with stab). Even if it doesn't KO, they still may not be able to move.
I also suggest another move: Will-O-Wisp. Halves Foe's attack and hurts them by a fair amount each turn. Also teach the Pokemon Calm Mind, which raises special attack AND special defence. Two-way-win there. Cover defences with will-o-wisp and Calm Mind, while raising special attack to an amount that makes Hex deadly. Also try Pain Split and Rest in the moveset, too. I hope this helps! -Jellohamster