Might be a bit late on the Volcanion discussion train but here goes anyway
I'm surprised no-one has mentioned HP Electric on this thing yet, I really think it's Volcanion's best 4th move on Choiced sets. It basically hits almost all of HP Grass's main targets (Keldeo Starmie Slowbro Suicune Slowking+Gyarados, and while it does miss out on Gastrodon and Sesmitoed, those two aren't exactly common, and being able to OHKO opposing Volcanion after SR with Specs / 2HKO with Scarf without running Earth Power (which you can't fit alongside Sludge Bomb if you want SE coverage on Waters) seems like a greater advantage, I doubt Gastrodon and Sesmitoed's combined usage will come anywhere close to Volcanion's, though they will probably increase in popularity due to Volcanion's presence in the first place, I guess we'll have to see.
I'd also mention HP Ghost as a viable move on this thing, for hitting Latis and Water-Psychics in one slot. Bug does this too, but Ghost also hits Jellicent, which is barely ever used, but since HP Bug has no real advantage over HP Ghost as far as I can tell, you might as well.
Specs and Scarf will almost certainly be the 2 main sets imo. Specs has already been discussed to death and I won't bother talking about even even more, but I think people are undervaluing Scarf a bit. Steam Eruption is obviously a very spammable move, and being able to just fire them off at high speed is really great, and though Volcanion isn't a perticularly fast scarfer, it still outspeeds most of the metagame, and being able to check stuff like Weavile, Diancie and Keldeo better, as well as get a second hit on stuff like Latios and Starmie before they heal up gives it an advantage over slower sets, especially on offensive teams.
As a whole, I don't think Volcanion will be quite as good as some people are hyping it up to be, at least as a standalone mon. Having both a SR weakness and low speed really limits the amount of teams it can fit on, you can point to stuff like Talonflame and Thundurus, but both have high speed which at least reduces the amount of pressure SR puts on them.
That being said, Volcanion seems to pair very well with some of the best Pokemon in the tier, the things that beat it are checked by Pokemon that are popular in the first place such as Pursuit TTar and Latios, and the support it provides itself really benefits a lot of dangerous threats. I can't wait to try out a Keldeo+Volcanion double choice water spam core which looks absolutely monstrous, especially with Pursuit support, and the fact that it's impossible to initially tell which will be Specs and which will be Scarf makes it just the more deadly.
So yeah, this thing will be a huge threat, primarily how well it synergies with popular threats. Can't say I'm too excited about having to account for yet another threat in teambuilding, but hey, at least it checks itself, right?