After messing around with it today and a little last night, I'd say Volcarona is mid-tier, although it's bordering low-tier for me. It requires a bit of work to tutor it Signal Beam (although that alone shreds a lot of stuff to pieces) and it doesn't do well against most of the gyms late-game. Slow experience growth sucks and Fire Blast comes a little later than one would like. BUT it does considerably well against the Elite Four. It can handle Grimsley bar Scrafty, can set up on Caitlin with Calm Mind (or just attack and use something else against Sigi), and can set up on Shauntal's Cofagrigus as well (you need a fire move though or Driftblim one shots you). It struggles against Marshall and Iris as pretty much every one of their Pokemon has a move that hits you super effective and having to use a bunch of potions is undesirable (a lot of Pokemon have trouble with that Hydreigon though and Volcarona handles it well). It also does pretty well against Team Plasma, so there's that. All things considered, it's certainly not terrible but it does require a lot of work. If you put the work in then it's going to be one of the most powerful Pokemon in the game. Purely because of the work required I can't say it's any higher than mid.
Mid/Low
Oshawott is high tier for me. Razor Shell is just so damn good early on and it learns a variety of good H/TMs (Surf, Blizzard, Dig, X-Scissor, Grass Knot). Admittedly, it has problems against Elesa and Burgh, but it can pretty much handle everything else on its own. Mine bashed through Roxie with Razor Shell (defense drops help a TON), Clay was no problem, Skyla is weak in general, Drayden's Druddigon and Flygon don't have enough power to break through and you can afford to miss a Blizzard on his Haxorus too (Dragon Tail, really?). Marlon has a problem with Grass Knot. I ran Surf / Razor Shell / Grass Knot / Blizzard and didn't have many problems. It can nearly solo the elite four if you've leveled enough. Plus, it's a starter so availability isn't a problem and its exp growth is acceptable.
Top tier/High tier
The Braviary you get in Route 4 is really a blessing (for White players, of course). It comes with Hone Claws and Wing Attack which shreds stuff to bits. You get Slash not too long after and eventually Return hits for a ton. You'll have Superpower by level up around the time you're at the Elite Four, which is nice for Bisharp and Krookodile. It's not amazing against the elite four or gym leaders as a whole and struggles against Achroma, and it also has slow exp gain. It definitely simmers down late game, I'd say. It's really good against standard trainers, though.
Mid tier
The next Pokemon was pretty terrible to be honest, but I raised him because he's one of my favorites (so I might as well tell you about my experience). Umbreon requires friendship evolution (ughhh), has virtually no power or speed (is there any silver-lining?...), and Eevee is only a 5% encounter rate. It doesn't even have a powerful STAB to counteract all of these negatives. On the bright side, mine evolved before it would learn Faint Attack so at least I had a decent STAB to work with. I found myself having to use Screech wherever possible, and it actually worked quite well. I taught it Payback, the final moveset being Work Up / Payback / Return / Moonlight. Work Up replaced Screech because you can actually sweep with this thing. Needless to say this was a weak Pokemon but it can tank very well.
Low
