I'd like to make a case for Volcarona being High Tier.
Volcarona: - High
Availability: Stationary in the PWT-accessible part of the Relic Castle, available after Clay at Lv. 35.
Stats: Higher Special Attack, Special Defense, and Speed than most anything available at that point, and it stays that way right through the end. The other stats are subpar, but it doesn't need them most of the time.
Typing: Fire/Bug STAB is brilliant offensively, only stopped by Chandelure. Defensively, it isn't anything special, but it has notable resists in Fighting, Ice, Bug, and Grass.
Movepool: It starts off with next to nothing and doesn't get much by level-up until later on, but luckily it has the Signal Beam tutor readily available. It's more than enough to get by until Route 13 for Psychic, then soon after it can pick up Fire Blast from Lacunosa. Decent filler exists in the form of Will-O-Wisp to cushion its physical Defense, and Quiver Dance at Lv. 59 can be learned before or during the Elite Four if you're willing to give it a couple extra levels. Other tutor moves include Giga Drain and Roost.
Major Battles:
Skyla: The only use it has here is Fire Spin against her Skarmory, and maybe the neutral Signal Beam against Swoobat. Otherwise, steer clear.
Hugh: Unfezant is tough, but the monkey is too frail to pose a significant challenge. Serperior is easy EXP, but Emboar and Samurott should be avoided. Later on, Bouffalant is bulky enough to take a hit and strong enough to OHKO you unless you burn it first. There are better options most of the time against him.
Zinzolin (All): It manhandles him every time. Fire Blast comes right in time to destroy his Cryogonal, and the Sneasel/Weavile is weak to both of your STABs and can only Night Slash you at best, which likely won't OHKO.
Drayden: It can spread burns and get strong neutral hits, but that's about it. The thing is, burn completely cripples Haxorus and Druddigon, and neuters Flygon. Watch out for Flygon's (and if in Challenge Mode, Druddigon's) Rock Slide, though.
Shadow Triad: All of their Pokemon are easily OHKOed, no questions asked.
Marlon: Sit this one out, you're not doing anything in here.
Colress: Avoid full-HP Magnets due to Thunder Wave, but otherwise, you walk all over him. Steels die to Fire Blast, Beheeyem to Signal Beam.
Kyurem-(B/W): It can get off a strong Fire Blast to take it down after it's been weaked, or in the case of Kyurem-B, Will-O-Wisp can cripple it. Kyurem-W's special attacks are taken fairly well with all that special bulk.
Ghetsis: Arguably the biggest use it has here is dealing massive damage to his Hydreigon with Signal Beam. Otherwise, it handles Cofagrigus and Toxicroak admirably with Fire Blast and Psychic, respectively. If you have Giga Drain, then Seismitoad goes down. Everything else is tough to crack, though.
Shauntal: Cofagrigus is massive setup bait if you have Quiver Dance. If you don't, then Drifblim and Golurk can be burned and/or hit with Flamethrower. Chandelure requires Psychic and some boosts to get by, however. At least she dropped the Jellicent, right?
Grimsley: The only problem here at all is Scrafty, who is bulky enough to take a Signal Beam and retaliate with Rock Tomb. If you can stomach setting up on Attract Liepard, then you're all set once it goes down.
Caitlin: Grab a couple Quiver Dances, wake it up, then win. In case you don't have Quiver Dance, she can still be mostly beaten with unboosted Signal Beam, but Sigilyph and her bulky Pokemon might survive a hit to do a little damage back.
Marshal: It can't afford to set up here, and Throh and Conkeldurr have Guts. Fortunately, it is probably faster than everything he has, and can mop up weakened Pokemon with Psychic or Flamethrower. If you don't intend to use it here, then it makes a great suicidal way to break Sawk's Sturdy.
Iris: Quiver Dance lets you survive Hydreigon's Surfs and easily KO it, then get a couple big hits on her other Pokemon. Without Quiver Dance, it's best used to revenge kill Hydreigon and maybe burn Haxorus. Almost everything she has can hit you super effectively, though, so be careful.
Additional Comments: It creams Team Plasma and most of the Elite Four, whilst never really losing steam earlier on. Sure, it has to sit out of a couple major battles, but they aren't particularly challenging, just unfriendly to Volc. My set going into the Elite Four was Quiver Dance | Flamethrower | Signal Beam | Psychic, and it swept all but Caitlin (who was demolished by Krookodile alone), Marshal, and Iris. Volc even helped out against the latter two by breaking Sawk's Sturdy and KOing Hydreigon. It doesn't even come late, really, despite only being available after the 5th Gym. There's still more than plenty to do in the game, and Volcarona really helped me out.