step up, snap ya back
In the grand scheme of things, Venomoth is slightly outclassed by Haunter in NU due to the latter's higher Speed, greater power, and better typing. Being one of the fastest Pokemon in NU with a reliable sleep move is Venomoth's biggest niche, since while Haunter is faster than Venomoth, it has to rely on Hypnosis. Access to Baton Pass and Stun Spore allows Venomoth to keep momentum, and its decent Special Attack means Venomoth can hold its own on the battlefield. However, Venomoth has no means of boosting its stats apart from Silver Wind and Berries, has to use its much lower Attack stat if it wants to make use of its STAB moves, and is anything but bulky. Despite these faults, Venomoth makes a capable lead and team player on offensively inclined teams that are looking for something a little different.
name: Lead Support
move 1: Sleep Powder
move 2: Psychic
move 3: Substitute / Hidden Power Fire / Stun Spore
move 4: Baton Pass
item: Leftovers / Petaya Berry
ability: Shield Dust
evs: 4 HP / 252 SpA / 252 Spe
Venomoth's biggest perk is that it's one of the fastest Pokemon in NU with a sleeping move, and this set aims to make the most of this fact. Venomoth works very well in the lead slot, as it outspeeds most of the other common leads, primarily Glalie and Seadra, and instantly puts them out of commission with Sleep Powder. Substitute allows Venomoth to safely Baton Pass out to a teammate and makes it far easier to activate Petaya Berry. Psychic and Hidden Power Fire give Venomoth great two-move coverage and allow it to nail a few of the common Pokemon Venomoth naturally has an advantage against such as Hitmonchan, Roselia, and Arbok for super effective damage. Stun Spore is a decent alternative in the third moveslot that helps support the team and can deter some dangerous Pokemon like Murkrow from freely switching into Venomoth. The last moveslot is dedicated to Baton Pass to allow Venomoth to keep momentum throughout the match and provide a fantastic way to catch Fire- and Steel-types like Flareon and Mawile looking to switch into Venomoth's attacks with a teammate like Diglett.
A Timid nature is needed to at least Speed tie with opposing Venomoth and Vigoroth and securely outspeed Seadra. A bulkier EV spread by moving Venomoth's Special Attack EVs to HP is certainly usable and allows Venomoth to more easily check the likes of Hitmonchan throughout the match. Using a Jolly nature, maximum Attack investment, and Venomoth's physical STAB moves is an option that allows Venomoth to hit Flareon and other specially bulky Pokemon like Kecleon far harder than Venomoth's special moves. Sludge Bomb also comes with the advantage of a high poison chance that can really mess with Pokemon like Sableye and Relicanth that otherwise have a relatively easy time switching into Venomoth's standard moves.
There are a variety of other moves that can be used in the third moveslot over the presented options that can tailor Venomoth into a better anti-lead depending on your team's needs. Hidden Power Electric can allow Venomoth to more easily anti-lead Seadra and the rare Huntail lead, as well as give Venomoth a strong option for Flying-types looking to switch into Venomoth for free. Giga Drain can similarly hurt Seadra and Huntail and deal an immense amount of damage to Relicanth, Sudowoodo, and Whiscash. Hidden Power Water deals a good amount of damage to the Fire- and Rock-types that give Venomoth some trouble, but unfortunately it fails to OHKO Magby and deals a paltry amount of damage to defensive Flareon. Hidden Power Dark is an option to catch Chimecho on the switch in, but it has little use outside of that target. Thief and Toxic are Venomoth's best choices to cripple the defensive Pokemon that otherwise have an easy time switching into it like Sableye and Flareon.
Vigoroth is the most pressing concern for lead Venomoth, as it is immune to Sleep Powder and will easily 2HKO Venomoth with Return. A sturdy switch-in to Vigoroth is needed, such as Relicanth or Piloswine; just beware to not become setup bait for Taunt + Bulk Up variants. As stated previously, Diglett pairs naturally well with Venomoth, as it can trap and eliminate a few of the Pokemon that have an easy time switching into Venomoth, namely Flareon, Mawile, and Metang. Venomoth fits best on offensive teams that need the immediate momentum provided by Sleep Powder and need Glalie's Spikes prevented from being set up from turn one. Good offensive teammates that pair well with Venomoth include Seadra, Plusle, and Pidgeot, as they can blow through a few of the Pokemon that are likely to switch into Venomoth and don't require setup to become an offensive threat.
While Venomoth's Attack stat is low, it isn't atrocious, and with Venomoth's high-power STAB moves, physical sets are not totally out of the question. Screech, when combined with Baton Pass, can be an infuriating set for your opponent to deal with, as they have to choose whether to stay in and brunt the incoming Sludge Bomb or Hidden Power Ground from Venomoth at -2 or allow Venomoth to scout their switch in with Baton Pass. A Choice Band set is less effective but still doable, and Venomoth has Baton Pass as an advantage over Golbat. Venomoth can put together a Sunny Day sweeper set with access to Solar Beam, but this is obviously just done better by Bellossom and Tangela. Finally, a Protect + Toxic set is an option, but Venomoth just doesn't have the bulk nor a healing move to make it work.
[Checks and Counters]
Countering Venomoth is not difficult, as it isn't exactly Earth-shatteringly powerful, but it is tough to pin it down because of Baton Pass allowing it to escape any troublesome situations. RestTalk Dewgong is probably the best overall switch-in to Venomoth, as it can absorb Sleep Powder and other status from Venomoth and takes a pittance from any of its moves apart from rare Screech sets, while in return it can easily 3HKO Venomoth with Surf and Ice Beam. Murkrow can switch into Sleep Powder and Psychic totally for free and easily outspeeds Venomoth and OHKOes it with Drill Peck. Other capable sleep absorbers such as Kecleon and Magcargo easily deal with Venomoth. Once Sleep Powder is out of the equation, countering Venomoth becomes an entirely easier ballgame. Chimecho and Flareon capably switch into anything Venomoth can throw at them, even Hidden Power Dark or Hidden Power Water, and easily OHKO with Psychic and Fire Blast. Rock-types such as Sudowoodo and Relicanth have to be wary of Giga Drain but will otherwise be able to tank Venomoth's moves and OHKO with Rock Slide. Crawdaunt has to be wary of Signal Beam but can otherwise easily switch into Venomoth and OHKO with Double-Edge. Finally, faster Pokemon such as Pidgeot, Raticate, and Haunter can easily revenge kill Venomoth after a layer of Spikes.