Smeargle @ Focus Sash
Timid - Own Tempo
252 Speed, 252 HP, 4 Defence
Smeargle is hands down the most reliable SmashPasser in the game, thanks to the fact that it's the only one who gets access to Spore. Passing a Shell Smash to a dangerous threat like Gengar or Mega Garchomp can be an an automatic win in many circumstances, since their usual checks and counters are going to be hard pressed to defeat them at +2/+2/+2. Magic Coat is great for bouncing back Taunt, Whirlwind, Thunder Wave, and various other status moves.
In general you should always Shell Smash first. This gives you the best chance at a safe switch thanks to Spore, meaning the only way they'll be able to hit you on the Pass is if they get a first turn wake-up. Keep Smeargle away from priority users and multi-hit users like Breloom, Scizor, Cloyster, and Mega Heracross, as they will ruin him. Smeargle appreciates hazard support, not for him, but to stop his SmashPass recipient from getting phased out or KO'd by Sturdy/Focus Sash users and getting a bit of extra chip damage on bulkier threats.
Other variants of Baton Pass Smeargle can be used but are generally more specialized (such as Geomancy + Cotton Guard on cancerous dEnIsSsS BP teams), as it does get every other potential set-up move in the game. Smeargle is also the only legal Passer of Ingrain, which is invaluable for full BP chains as it stops opponents from phasing you with Roar, Dragon Tail, etc. as well as providing some passive recovery.
In formats where Baton Pass is restricted, Smeargle is essentially rendered useless. It can make an okay hazard lead, but is largely outclassed by other hazard setters. Its only niche is that it can run any and all hazard moves, something no other Pokemon can boast. However, due to its pathetic defenses and average Speed, it'll rarely get more than 2 layers up and may struggle to get more than one, and it won't be surviving long enough to reset hazards later in the match if they get spun or Defogged away. Spore + Imprison + Transform is fun but a complete gimmick that should never be used on a serious team outside of 1v1.