Just finished the E4 earlier today and would like to share experiences with a few Pokemon
Although it starts off with a horrendous moveset, its stats are incredibly high at the time of capture. You can also get Signal Beam easily on it, especially since there's a guy who gives you 10 Red Shards, and it will deal massive damage to a lot of things. Unfortunately, Volc won't have anything new for a while, until Undella town where you can pick up TM 38, Fire Blast, from the Pokecenter, and TM 29, Psychic, from the route above it. As shaky as Fire Blast is (5 PP and 85% acc; I PP upped mine lol), it provides Volcarona with a much needed fire move (go die, Fire Spin and Ember), and by then you'd have all the coverage you need. Volcarona, sadly, doesn't fare too well against the last three gyms, but once you get it to Lv.59 for Quiver Dance it will absolutely murder the Pokemon League almost singlehandedly (beats Dark/Ghost/Psychic/Iris (her Aggron doesn't have Sturdy) without a sweat, since it can set up on their first Pokemon). Volcarona imo is at least Mid tier, but I can see it being in High.
Set after E4's completion: Quiver Dance/Fire Blast/Signal Beam/Psychic @ Leftovers. I could've gone Flamethrower but I already pp upped fire blast so i'm not wasting that.
I don't need to say much about this but it's a strong Pokemon all game long and walls soo much stuff with its 13 resistances. Decent coverage too for something with 130 base SAtk. Sturdy + Thunder Wave offers some backup utility if ever needed. Also singlehandedly destroys Kyurem-B. Top tier
Set after E4: Thunderbolt/Signal Beam/Flash Cannon/Thunder Wave @ Shell Bell (lol).
Comes at a reasonably high level in Undella (up to 40). Unfortunately, that's pretty late, but its coverage by the end of the game (and its Speed), is amazing. A simple Surf/Psychic/Ice Beam/Thunderbolt (SPIT) @ Expert Belt can clear trainers and much of the Elite 4 with ease. Water Stone can be picked up in Route 19. I'd say this one is Mid tier.
I've had moderate success with Zubat as well, really only due to the sheer strength of Acrobatics. Unfortunately its physical movepool is pretty trash (Acrobatics/Return/Fly/...Bite?). No matter, Acrobat is usually all you need to spam. Also, Acrobatics is just cool.
My full team at the end of the game: Volcarona/Starmie/Virizion/Magnezone/Crobat/Haxorus.
I expect old titans like Sandile/Darumaka/Drilbur to still perform well in this game, even though I didnt use them this time. Unfortunately Drilbur isn't available really early on anymore but the best ones are in Chargestone cave anyways (also the cave at the bottom of pwt).