So I just used Vivillon in a few battles. It's good. Damn good, even with Leftovers as it's item, which is what I did.
In my opinion, it's even better than Venomoth as a sweeper, but that's up to interpretation.
Definite RU/UU contender.
(The fact that it learns Hurricane... is so good)
Yeah, it's x4 weak to Rock, but if you use good support, it's a great team member.
As soon as I heard Vivillon got Hurricane (back on release day) I knew that it would very likely get more usage than both Lilligant and and Venomoth, who are similar quiver dancers. While having 90 base speed instead of 100 (and special attack too, but that's less important) is rather unfortunate, it has a few things over both Lilligant and Venomoth, the first, and probably most important, is a much better attacking stab. Hurricane with compound eyes is very reliable, and very powerful, after one boost, you have a 56.25% chance to OHKO after Stealth Rock to Garchomp, which, while isn't a premier wall or anything, but by comparison, venomoth would only be doing 71.22 - 84.35% while Lilligant is stuck doing 74.3 - 88.26% when Modest. The problem those always had wasn't walls, but having to require multiple boosts to even get past normal attacking threats. Throw in the fact that all the things that resist flying moves happen to be weak to ground, and it becomes much easier to lure away those counters (it's a shame Hidden power got nerfed). Throw in buffs to defog, and some people using sticky web over rocks, and being dual weak to SR isn't as bad as it was in previous generations.
However, the chance of it being OU are slim, since 2 boosts are preferred to guarantee kills, it has rather weak defences, doesn't really have too much in the way of going though pokemon that resist it and more than anything, Volcarona exists, who is essentially stronger, faster, bulkier, and isn't afraid of common wall types (steels) but lacking sleep powder and trades flying STAB for a Fire STAB. STAB hurricane is rather useful though, as it does give 50% chance to KO Terrakion at +1 (although a scarf one will still outspeed you), something that Volcarona can't boast.
Doing calculations, a lot of things are rather scared of +1 LO Vivillon if rocks are up, of the more bulky pokemon, these are OHKO with rocks up with either Hurricane or Bug Buzz: Fortress, Bulky DD Gyarados, Donphan, Gliscor, Kyurem-B,(87.5%), Landorus-T, Latias, Latios, Salamance, Volcarona, Mamoswine, Celebi, Reuniclus, Ninetails and Conkeldurr.
And other notes, Hippowdon has a 43.75% after rocks, Jellicent, Tentacruel and Vaporeon are easily set up on, if at full HP Vivillon can get to +2 on Rotom-W and Politoad and live (if at full) and OHKO in return with bug buzz and hurricane respectively. It can KO Scizor at +1, but since it has 56.25% chance of being KO'd by bullet punch, it probably won't be wise to risk it. Dragonite after rocks is KO-able in all variants bar the special defensive one (defensive ones do however only have a chance to be KO'd after rock, not guaranteed), but dat extreme speed.
Vivillon isn't "a shitty Pokemon" by any means, although it still probably won't be featured in OU too much (I should have really done calcs on pokemon in lower tiers, but, meh).
