Volcarona B- ----> B+, or at least B.
The discussion on Volcarona has been brought up by
AM a while back (right after the update post) so I'm here to elaborate a bit. Volcarona is a REALLY underrated threat that can easily sweep unprepared teams or even prepared teams with the right support, this thing being at B- is really laughable. Looking at Volcarona, it doesn't have the best stats or typing, but it has one thing that makes it extremely viable and able to crush Pokemon like it's not even funny, it's Quiver Dance. After 1 Quiver Dance, it outspeeds the entire Scarf-less tier and even outspeeds up to Scarf Landorus-T, missing Scarf Keldeo, Terrakion, Garchomp, and Lati@s, which are almost ancient (Except Scarf Keldeo, but it's not as common).
This thing's speed is very great, sitting at 100 base speed enables Volcarona to outspeed the most common fastest Pokemon at +1, Scarf Jolly Landorus-T, without even having to fully invest in speed (Granted, requires a Timid Nature). It also outspeeds many threats such as Jolly Excadrill, Jolly Breloom/Bisharp, and even Jolly/Naive Mamoswine at +0.
The second thing that comes to mind is this thing's good coverage. Having access to a strong STAB in Fire Blast, a decent one being Bug Buzz, and other coverage (non-STAB) moves such as HP Ground, Giga Drain, or even HP Rock is only resisted by a selected few depending on the combination. Even when resisted, a strong Fire Blast at +1 does a lot. For example, if running Fire Blast with Giga Drain / HP Ground combo, Fire Blast has a very high chance (~95%) to 2HKO Latios after rocks at +1.
Volcarona is best used after the opponent's team has been crippled a bit, after that, a single QD and it's basically GG.
Here are a couple of replays showcasing Volc doing a LOT of work (in fact winning the game):
http://replay.pokemonshowdown.com/ou-205108885 - After I chocked and used Defog on the incoming Bisharp, the opponent had the advantage at the beginning and was keeping momentum. However, after some smart switching and predictions, the opponent chokes by using SR with Ferro on Latias while I switch to Volcarona. I QD, and then it's literally gg.
http://replay.pokemonshowdown.com/ou-205339747 - This one is pretty high ladder, after I weakened Azumarill and made sure it was not AV or CB, I baited the opponent into Hydreigon and after it Draco'd twice, it was -4. Using this opportunity I sent Volcarona and with one QD the game was over, and the opponent knew it.
Of course, Volcarona has some universal consistent counters, mainly being Mega Altaria, Chansey, and Dragonite (Talonflame is a very solid check, but is not consistent). It also has some lackluster Defense, making it susceptible to priority, mainly Azumarill and Talonflame. Azumarill only counters it if it's 1) good health and is AV or 2) is CB and has Aqua Jet (ofc). Talonflame on the other hand just outright roflstomps on Volcarona. Not only that, but Volcarona does indeed have 4MSS, not being able to run all its coverage moves will make vulnerable to certain Pokemon, such as Heatran if you're lacking HP Ground. But overall, through it's great ability to sweep/clean, good coverage, and good speed tier warrants it a rank of at least B, even though I support it to B+ more.