Venomoth... Okay, let's see. This Bug/Poison type was fairly forgettable in the last few generations. It was basically a generic Bug type. 4th gen, however, brought it a great ability in Tinted Lens. For those who don't know, Tinted Lens doubles the power of "not very effective" moves, so a 2X resist becomes a neutral hit and a 4X resist becomes a 2X resist. That makes its lackluster STAB coverage less of an issue. Even with Tinted Lens, however, it was left in the shadows of fellow Tinted Lens user Yanmega, who is faster and more powerful.
5th gen brought a huge change for Venomoth, however: Quiver Dance. This amazing boosting move is spread to only a few Pokemon (all Bug types, with the exception of Lilligant), and it boosts Sp. Attack, Sp. Defense, and Speed one level. So its like Calm Mind with a Speed boost thrown in. Venomoth's stats in these areas are all decent or better, so it can become quite threatening once it gets a few up. Venomoth also got the Hidden ability Wonder Skin, but with an amazing ability like Tinted Lens, there's little reason to bother.
Two other Pokemon get both Tinted Lens and Quiver Dance: Butterfree and Mothim, but neither of them is as fast as Venomoth (although Mothim packs a bit more power). Furthermore, those two are Bug/Flying types, and as such have that awful 4X Stealth Rock weakness, whereas Venomoth only has a 2X weakness, so it can switch in a bit more easily. So, being the only Bug/Poison type with Quiver Dance and Tinted Lens gives Venomoth a niche. It can't really compete with fellow Quiver Dancer Volcarona and its huge power, but Venomoth has its own niche.
This will probably end up being Venomoth's most common and most effective set this generation:
Item: Leftovers
Ability: Tinted Lens
Nature: Timid (+Speed, -Attack) or Modest (+Sp. Attack, -Attack)
EV Spread: 252 Sp. Attack, 252 Speed, 6 HP
Item: Leftovers OR Life Orb
Sleep Power
Quiver Dance
Bug Buzz
Sludge Bomb/Hidden Power Ground/Baton Pass
A simple, yet effective set. Put your opponent to Sleep with Sleep Power, then use the free turn(s) to set up. There are only two offensive moves on this set (and only one if you run Baton Pass), but that's what Tinted Lens is for. It makes coverage less of an issue.
Bug Buzz is a given. 90 base power, a chance to lower Sp. Defense... Yeah, that's good. Just watch out for Soundproof Pokemon, who are immune to it. The second move is there to compliment Bug Buzz. Sludge Bomb is an additional STAB, although its offensive coverage is lackluster (its only supereffective on Grass Pokemon). Like Bug Buzz, Tinted Lens makes this less of an issue.
Hidden Power Ground murders any Heatran without an Air Balloon, which is important, because otherwise it walls this set. Not that it's likely that you'll see Venomoth and Heatran go head-to-head, but still. It also smacks a number of other counters hard, like Chandelure.
Baton Pass is another niche move Venomoth gets. Besides Masquerain, its the only Quiver Dancer who can Baton Pass. If Venomoth is faced with a threat it can't handle(like the aformentioned Heatran and Chandelure), Baton Pass to another Pokemon. Almost any Pokemon can benefit from Quiver Dance in some way, shape, or form, so it's a pretty worthwhile move.
Nature choice is simple. Timid if you want more speed, and Modest for more power. Timid is probably preferable, though, so you can get more Pokemon to sleep before they can take you out. As for item choice, pick the Lefties if you want a little more bulk, and Life Orb is all about power.
I guess you could kinda-sorta run a defensive set with Roost, but Venomoth's stats just aren't up to the job. Quiver Dancing is what it does, and it does it well. It's got the stats, it's got the ability, and it's got the moves. And God help you if it Baton Passes two or three Quiver Dances to an even more powerful ally.
