903 Sneasler

Poison Jab benefits from STAB to deal respectable damage, but its coverage is rather limited. Shadow Claw can be chosen for its typing which is largely unresisted by most Pokemon. 

Close Combat is always recommended for its powerful STAB coverage, despite the harsh self-debuff. X-Scissor costs the least amount of energy in the game, allowing it to be spammed frequently and bait shields. Aerial Ace is not recommended due to its unimpressive stats, unless Flying-type coverage is needed. 

Sneasler's high ATK stat results in poor bulk for Great League. Relatively fast access to Close Combat lets Sneasler threaten OHKOs against various Pokemon in the meta, not just those weak to Fighting-type damage. Successful baits X-Scissor will significantly improve Sneasler's matchups, but this can lead to inconsistent results. If you like the high risk and high reward style, Sneasler might be for you. 

Much of Sneasler's performance in Ultra League will come down to whether it can consistently bait shields with X-Scissor, which seems to be the common theme. Sneasler is the faster and more polarizing version of Perrserker, with the faster X-Scissor shield bait while also being more reliant on it. Despite having a major and consistent advantage against Trevenant, Sneasler's lack of bulk is notable enough to cause it to lose if it gets hit by one Shadow Ball. 

Without a CP cap restriction, Sneasler shines best in Master League with its high ATK stat. Poison Jab is mandatory in order to overcome Togekiss (and other Fairy Pokemon), but Shadow Claw tends to be better against the overall meta. It should be noted that Sneasler should avoid anything that flies, for its obvious coverage vulnerability.