Red/Green/Blue/Yellow
Trainer: Champion Blue, Levels 59-65
Well no surprise here, he's the final boss and there are no end game trainers
Pokemon: Mewtwo, Level 70
Likewise, it's the Final Pokemon after all
Gold/Silver/Crystal
Trainer: Red, Levels 73-81
Again, kind of a no-duh
Pokemon: Ho-oh/Lugia, Level 70
This one's interesting because it depends on your version. You can get both in GS versions, but the "second" one will be at level 70 in the post game. This also means Crystal's highest leveled Pokemon is actually 60, not 70 but it wasn't worth giving it its own category.
Ruby/Sapphire
Trainer: Steven, levels 55-58
RS had more trainer rematches, but none of them reach the heights of Steven, even fully powered Wally.
Pokemon: Rayquaza, level 70
As expected, the "Mewtwo" of the game.
Fire Red/Leaf Green
Trainer: Rematch Champion Blue, levels 72-75
FRLG was the first to introduce the concept of E4 rematches, so he gets to rein supreme again.
Pokemon: Mewtwo, level 70. Ho-oh & Lugia, level 70
If we include events, the Ho-oh & Lugia you get at Naval Rock are also level 70. You know it's funny, this was apparently released at various festivals in America but I never remember hearing about it unlike the Aurora Ticket.
Emerald:
Trainer: Steven, levels 75-78
Yeah it's still Steven, his fight in the Post game.
Pokemon: Kyogre, Groudon, Rayquaza, level 70. Ho-oh & Lugia, Level 70
Since the other 2 are now post-game, they also join the level 70 train. Rayquaza they didn't actually lower the level of so...
Emerald could also do the Naval Rock event, but I'm less sure if this was released or where....
Diamond/Pearl:
Trainer: 20 Hall of Fames Barry, levels 68-74
I wonder how many people ever did this? Incidentally, DP had no such level raises for the e4 so Cynthia's levels remained the same forever.
Pokemon: Giratina, Regigigas & Heatran, level 70.
Cresselia's the only post game legendary Pokemon not in the level 70 club. There is, of course, the Level 80 Arceus event, but this was never released ever, so I'm not including it here.
Platinum:
Trainer: 20 Hall of Fames Barry, levels 79-85
The e4 actually do get a level bump this time, for beating Stark Mountain, but full powered barry goes all the way
Pokemon: Magikarp, level 100
thanks resort area
Lowest Leveled Pokemon: Regigigas, Magikarp, level 1
by law we have to point this out
Heart Gold/Soul Silver:
Trainer: Red, levels 80-88
Red is extremely petty.
Pokemon: Lugia, Ho-oh, & Mewtwo level 70
Same as GS but also since they restored Kanto, Mewtwo returns.
Black/White:
Trainer: Alder, level 75-77
I actually had to check a couple trainers to make sure but yes the true champion fight is the strongest available.
Pokemon: Kyurem, level 75
Completely forgot that Kyurem was level 75. I thought it was level 70 again, just like all the other "Mewtwos" and just like Landorus.
Black 2/White 2:
Trainer: Challenge Mode Benga level 85 or Challenge Mode Iris, levels 81-83
I included Iris because of a specific quirk: Benga only has 3 Pokemon but Iris has a full 6. To this point that hasn't really mattered at all, everyone on this list has 6 Pokemon and happened to have the highest levels. But here it didn't seem right to fully write off Iris.
Pokemon: Reshiram/Zekrom & Kyurem, level 70
See now we're back to 70. Reshiram/Zekrom are VE, of course. Did you know the Heatran & Cresselia you get is level 68? That's so goofy.
XY
Trainer: Black Write of Challenge Battle Chataeu Diantha, level 80. Post Game Calem/Serena, level 66-70
Like with BW2 I felt the need to list both of these. The Chataeu teams are only a team of 4, where as Calem/Serena are a full team of 6. They just barely have higher levels than normal Diantha.
*sigh*....a Z version probably would have had "proper" rematches....
Pokemon: Zygarde, Mewtwo, Articuno/Zapdos/Moltres, level 70
I was a little taken aback when I saw the birds were also level 70. I figured Zygarde & Mewtwo would, but it's funny to see the other trio be level 70 as well.
Omega Ruby/Alpha Sapphire
Trainer: Battle Maison Full Power Wally, levels 79-81
This is the final rematch of Wally you unlock at the Battle Maison, putting him 2 levels ahead of Rematch Steven. As a reminder he also has all his pokemon with full items and everything. Wally got swole.
Pokemon: Deoxys, level 80
I actually forgot that the Delta Episode Deoxys was so high a level! The highest seen to this point if we ignore Arceus.
Sun/Moon
Trainer: Red, levels 65-70
I had this whole long spiel about Kukui until I remebered Red and it's always red. Thoug hit is interesting he doesn't breach 70...the Alola games are pretty interesting in that while they fully experimented with proper EVs and so on on every boss trainer, plus the idea of totems and such, they played the level curve """lower""" than you might have thought. Maybe their way of balancing out the increased stats from EVs, eh?
Pokemon: Necrozma, level 75
I think they made it level 75 so Guzzlord could be level 70.
Ultra Sun/Ultra Moon
Trainer: Red, levels 69-74
They literally made him higher because there were 3 other competitors that got to level 70. I had a whole OTHER spiel here and its ruined beacuse of Red.
Pokemon: Necrozma, level 65
Every other single legendary is level 60 at most, so Necrozma gets to be special.
Let's Go
ITS RED OKAY ITS RED AGAIN LEVEL 85
Straight 85s this time, no slight step ups in levels or anything. 6 members, all 85. By far & away the strongest trainer in the game, though a few master trainers with their single pokemon do get pretty high.
Pokemon: Mewtwo, level 70
duh
but hey did you know you can encounter the birds as low as level 3? Dragonite too. That's fun.
SWSH
Trainer: Final Battle Hop, 68-70
Post-game Tournament Leon actually ties with Post-game Tournament Hop, but Final Battle (the one at the end of the post game) Hop has a full-team battle so i gave the edge to him. SWSH is so bizarre when it comes to its rematches, for some reason every single trainer only has 5 pokemon, even the two that we KNOW have full teams!
Pokemon: Zamazenta/Zacian, level 70
As per usual.
SWSH, but with DLC!
Trainer: Post-Galarian Stars Tournament Mustard, levels 78-80
After completing the GST, both Mustard & Leon gets a permanent level boost to level 78-80. However, Leon's rematch team is still only 5 pokemon but the Mustard you can freely fight at the Dojo has 6 Pokemon. Therefore, I gave the edge to him.
Pokemon: Raid Regigigas, level 100
Not going to include any of the actual "event" raids, because I don't care to look them up but I imagine a couple of them were level 100.