Nihilego @ Focus Sash
Ability: Beast Boost
Nature: Timid
EVs: 252 HP / 4 SpA / 252 Spe
- Stealth Rock
- Toxic Spikes
- Magic Coat
- Clear Smog
Nihilego is my favorite lead to use, because it's one of the few Pokémon that gets access to both Stealth Rock and Toxic Spikes. If you manage to knock out a Pokémon, you get a Beast Boost in Speed, which may or may not be helpful.
- Stealth Rocks because it's a lead.
- Toxic Spikes because it's a lead. If you don't need T-spike support, switch this for another move.
- Magic Coat bounces back opposing hazards, taunts, thunder waves, etc.
- Clear Smog shuts down any attempt for the opponent to boost their stats. It's also perfectly accurate, so evasion boosts don't matter.

COUNTERS:
-Steel types by far are the biggest counter (especially Steel type setup sweepers) because the only attacking move does no damage to them, plus Nihilego is weak to Steel.
-Faster leads, especially physical attackers, can knock out Nihilego before it can set up all the hazards.