I think the groudon suggestion is my favourite so far
Edit: Another suggestion
Tyranitar @ Chople Berry
Ability: Sand Stream
EVs: 252 HP / 252 SDef / 4 Atk
Careful Nature
- Stealth Rock
- Payback
- Pursuit
- Stone Edge
Nothing outright counters this pokemon as a lead except maybe groudon, it anti-leads a lot of other possibilities bar smeargle really, provides stealth rock, and is a decent check to mega-gengar and mewtwo forms. It also doesn't massively inhibit teambuilding as anything from full stall to bulky offense (maybe even HO?) appreciate ttar.
				
			Edit: Another suggestion
Tyranitar @ Chople Berry
Ability: Sand Stream
EVs: 252 HP / 252 SDef / 4 Atk
Careful Nature
- Stealth Rock
- Payback
- Pursuit
- Stone Edge
Nothing outright counters this pokemon as a lead except maybe groudon, it anti-leads a lot of other possibilities bar smeargle really, provides stealth rock, and is a decent check to mega-gengar and mewtwo forms. It also doesn't massively inhibit teambuilding as anything from full stall to bulky offense (maybe even HO?) appreciate ttar.
			
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