If you want to go big, you could run Focus Sash and minimum investment in both defenses, to make Counter or Mirror Coat a guaranteed OHKO. Although, on average, Leftovers is probably better, since Sash makes you extremely susceptible to hazards and status. The moveset would probably look like:
Wobbuffet @ Focus Sash
Ability: Shadow Tag
EVs: 252 HP / 252 Spe
Hasty Nature
IVs: 0 Atk / 0 Def / 0 SpD
- Counter
- Mirror Coat
- Safeguard
- Destiny Bond
Honestly, Wobbuffet really isn't that great of a Pokémon. If it had more speed, it could make more use of Destiny Bond, Safeguard, and Encore, but instead you really just have to rely on Counter and Mirror Coat. At least it can serve as an almost guaranteed revenge killer if your opponent has a stat snowball that's out of control. Focus Sash makes for a more reliable one-time killer, while Lefties gives you a chance to KO more Pokémon but is vulnerable to set-up sweepers (although Encore answers that if they haven't already set-up by the time Wobbuffet comes in). Overall, though, I'd probably say Leftovers is the superior set:
Wobbuffet @ Leftovers
Ability: Shadow Tag
EVs: 252 HP / 4 Def / 252 SpD
Bold / Calm Nature
IVs: 0 Atk
- Counter
- Mirror Coat
- Encore
- Destiny Bond / Safeguard
Unfortunately, investing into the Defenses is a double-edged sword for Wobbuffet, because you'll be doing less damage with Counter/MC, but it's necessary for the Leftovers set because you want to take more than one hit.