Scenario 1: It can go either way. I don't believe one Champion is VASTLY superior to another, but rather they're all equals or at the very least are on the same level with a MINIMAL difference between them. One has to look at the performances of each Champion's Ace.
Lance's Dragonite (Last time we saw it: Dragon Claw, Twister, Wing Attack, and Hyper Beam)
Beat a TR officer's Fearows (I think it actually struggled which doesn't look good for a Champion, I have to endure OS to check), but in Hoenn we see Lance's Dragonite clashing with Kyogre for a little while and that's the last we see of it.
Steven's Metagross (Meteor Mash, Psychic, Flash Cannon, and Protect)
Even in Base Form, it casually stopped a Flamethrower from Alain's Bulldozer Charizard, and after Mega Evolving it straight up tanked Charizard's best attack with no damage at all. Though to be fair, this is BEFORE Alain's Charizard got it's insanely ridiculous feats. Given how Alain's Charizard and Ash's Greninja are more or less portrayed as equals or Charizard is the slight superior, I'm convinced in a rematch Metagross wouldn't beat Charizard without Mega Evolution. Though in Base Form it did floor Primal Kyogre.
Cynthia's Garchomp (Last time we saw it: Draco Meteor, Dragon Rush, Brick Break, and Stone Edge)
True to her species, Garchomp is the stuff of nightmares, darn near every battle she's been in has been a one sided beatdown in her favor with the only Pokémon challenging her are Flint's Infernape who manhandled half of Ash's team with ease, but even then Flint's Infernape couldn't really faze her with Garchomp shrugging off his hits and immediately retaliating with one of her own. The other being Caitlin's Gothitelle whom she stalemated with, but Garchomp was definitely the stronger Pokémon between herself and Caitlin's Gothitelle. Gothitelle scored multiple attacks on Garchomp who shrugged them off, but one Dragon Rush and Gothitelle was struggling to get back up. She did fight the Forces of Nature which is impressive.
Alder's Bouffalant (Head Charge and that's all we've seen it do)
Alder hasn't done much, the only battles we've seen from Bouffalant were against Ash's Pikachu where it was shrugging off Pikachu's hits with no damage before attacking it's own trainer who was asleep and giving a no holds barred beatdown on Trip's Serperior.
Diantha's Gardevoir (Shadow Ball, Moonblast, Psychic, and Reflect)
Pretty much got the "Garchomp Treatment". She's seemingly untouchable, 1-2 shots her opponents, though she doesn't tank hits like Garchomp does, Diantha's Gardevoir BY A MILE is the fastest Pokémon out of the bunch here, seemingly teleporting away from attacks, but in reality is simply flash stepping. Moveset doesn't matter much in the anime, but Diantha's Gardevoir definitely has the best moveset that covers her weaknesses and asserts her strength. Some people give Gardevoir flack for her battle with Ash's Greninja, but I'm sure Diantha was holding back, she's primarily using Shadow Ball and the one time she told Gardevoir to use an effective attack, the shockwave alone blew Greninja off of his feet into a forest and she told Gardevoir to use her full power after TRio tried to steal her. Though we've seen that even in Base Form, Gardevoir can still react to Greninja's blows, dodge them, outspeed him, and block his attacks. Not to mention during the Team Flare Arc. Steven and Diantha are more or less portrayed as equals.
From all of this I'd say: Garchomp >= Metagross = Gardevoir >= Dragonite >= Bouffalant. Though I'd see either Gardevoir, Garchomp, or Metagross taking it.
Scenario 2: Again, Steven and Diantha are portrayed as equals, I'd see Steven's Base Metagross struggled with Ash's Greninja more than Gardevoir did because part of the reason why Gardevoir could last against Greninja the way she did was before of her speed, and we haven't see Metagross move that fast. Metagross has better bulk tanking Charizard's best attack with no damage, opposed to Gardevoir who was floored by Greninja's best attack (although she immediately got back up). I guess I lean more in favor of Gardevoir due to speed and Diantha's battle style being better than Metagross and Steven's.
Scenario 3: Again, I can see either Metagross, Garchomp, or Gardevoir taking this. Steven outright said ME, makes Metagross faster and Metagross has some serious bulk tanking Blast Burn. Garchomp is already a tank, but ME may reduce her speed, but give her physical strength a boost. Gardevoir has the speed, a moveset that can handle both of their typings, while reducing damage from physical attacks. Hard to say, but I'll go with either Metagross or Gardevoir on this one.
