Honestly, most champion's teams are pretty good. The exception is Wallace, with arguments for Alder, Lance, or Hau also being bad. Ignoring rematches:
Blue: Pidgeot, Alakazam, Rhydon, Gyarados, Arcanine, Exeggutor(sub in starter). So good that it became his official team. Lots of powerhouses given the generation, but even now they can show up as a high-end fight and be reasonably challenging(granted, gen 1 bias for improvements helps with that).
Lance(GSC/HGSS): Gyarados, Charizard, Aerodactyl, Dragonite x3. Ugh. Thing is, this is a scary team. And the movesets are actually good. But it doesn't have Kingdra, it doesn't have Tyranitar, and it does have 4 mons weak to ice and 3 weak to electric, so you can just sweep. It's mono-flying, not mono-dragon, which is a constant Lance issue.
Steven: Skarmory, Claydol, Aggron, Cradily, Armaldo, Metagross. More a mineral specialist than a steel specialist, but they're powerful mons with good coverage and no real shared weaknesses. All bulky enough to prevent just sweeping through, and with Toxic to prevent setup spam, it's about as good as you can do with the constraints of the character.
Wallace: Wailord, Tentacruel, Ludicolo, Whiscash, Gyarados, Milotic. Just what this region needed, more water. They're all good mons, and given they share types there's plenty of ways to avoid electrics or grasses, but mono-water after the player has beaten a bunch of water routes, a water gym, Team Aqua, and Kyogre is just sad. There's not even an attempt to make a viable rain team, which would at least have been interesting.
Cynthia: Spiritomb, Roserade, Gastrodon, Lucario, Milotic, Garchomp. The team and champion everyone talks about. So good they don't bother changing it for rematches. The first non-specialist champion since Blue, starting a trend of them. Good coverage, strong mons, and it tests your team in multiple ways. Perfect, no notes.
Alder: Accelgor, Bouffalant, Druddigon, Vanniluxe, Escavalier, Volcarona. All new mons is nice, the ones he chose are not. 3 bugs, no pseudo-legendary, and some baffling choices...I'm not surprised they replaced him for BW2
Iris: Hydreigon, Druddigon, Archeops, Aggron, Lapras, Haxorus. Dragon-focused, but with minimal type overlap, great coverage, strong mons, no repeats. It's not memorable, in that you're likely to remember it as Dragon spam and very offensive, but it's strong. And it actually has 2 ice resistances. Lance, learn from her.
Diantha: Hawlucha, Tyrantrum, Aurorus, Gourgeist, Goodra, Mega-Gardevoir. 5/6 mons are new to the region(and MGard is arguable), mix of physical and special offense with great coverage on each, some reasonable bulk, it's a solid team. It loses to good setup and this gen as a whole is relatively easy, but I don't think there's obvious ways to improve it.
Kuiki: Lycanroc, Ninetails-A, Braviary, Snorlax, Magnezone, Starter. Again, lots of fast powerful mons. But lots of non-Alola mons, no exploiting anything like Aurora Veil or similar, and it's mostly mons that go down fast. There's not even any subtle or strong theming to justify it. Of course, rival trainers were never the challenge of this gen. Fine, but could be better with a bit of effort.
Hau: RaichuA, Eeveeloution, Tauros, Noivern, Crabominable, Starter. It's not better. Still decent mons, no shared weaknesses, but also no theming, no major strategy, and not bulky enough to avoid being crushed. Given the higher power of the player in USUM, this is just not good enough. Needs a complete rebuild of Hau's team throughout the game to fix, though, so I get why they didn't do that. Oh well, title defense and Ultra-Necrozma are the real challenges of this game.
Leon: Aegislash, Dragapult, Haxorus, Charizard, Starter, Rando. I like this team because of what it says. This is a team you as the player might make. Pseudolegend, two broken mons, starter, Gen1 gift mon, fill out the type coverage. Going up against it, more than any rival, is supposed to be like going up against yourself. Every non-starter has perfect coverage, some of them are reasonably bulky, and there's very little way for the AI to screw it up. It can only be so challenging because Dynamax, but I really like this team.