Hmm. Never really thought about them before. Let’s see…
So, their teams are as follows (movesets and items are generally an improvement on their basic lineups):
Red: As his Heart Gold/Soul Silver team
Blue: As his Heart Gold/Soul Silver team, but swapping Pidgeot for Aerodactyl
Lance: Salamence, Haxorus, Kingdra, Flygon, Hydreigon, Dragonite
Steven: As his Emerald team, but swapping Skarmory and Claydol for Archeops and Excadrill
Wallace: Sharpedo, Swampert, Walrein, Starmie, Ludicolo and Milotic
Cynthia: As her Black 2/White 2 casual team, but swapping Milotic for Roserade
Alder: Reuniclus, Chandelure, Krookodile, Conkeldurr, Braviary and Volcarona
So, Red doesn’t change at all, and Cynthia just reverts to another of the Pokémon she used on Platinum. It’s odd that Blue would replace Pidgeot, who is his strongest Pokémon on Heart Gold and Soul Silver, but throwing in Aerodactyl instead certainly makes for a more menacing line-up, and fits his obsession with rare Pokémon (although I seriously question his decision to teach it Fire Blast, particularly in a Choice Band set…). The changes to Steven are, likewise, straightforward; Archeops over Skarmory fits his interest in rare stones and fossils; Excadrill over Claydol gives him another Steel-type to replace Skarmory and helps him to dig up cool new rocks. I’m really not sure about Head Smash on Archeops with Defeatist to worry about, but I suppose it combos decently with Sitrus Berry and Acrobatics, and I’m glad to see the back of that bat$#!t special attacker Aggron set he used in the third generation games.
Wallace changes out most of his team, keeping only Ludicolo and his signature Pokémon, Milotic. Wallace doesn’t have much of a unifying theme to begin with other than being a Water trainer; his battle philosophy revolves around grace and elegance, but all that translated to in Emerald, as far as I could tell, was using Pokémon who were a pain to kill, like Amnesiac Whiscash and Double Team Ludicolo. His new team seems generally more heavy on offence, mostly due to the presence of Sharpedo and Starmie, but I don’t think that makes it any more or less appropriate to him. I’m not keen on the fact that his Swampert is a special attacker, but I suppose it’s not as bad an idea as a special Aggron.
I actually don’t like Lance’s all-Dragon team much, simply because I was fond of the way his original Gold and Silver team worked around the single-type limitation while still making him very obviously a ‘Dragon Master.’ It made him stick out a bit amongst all the other single-type Gym Leaders and Elite Four members. Multiple Dragonite is obviously a no-go, but I think I would have gone with a compromise team, replacing Haxorus and Flygon with Aerodactyl and Charizard, and maybe Gyarados over Kingdra too. Also, Solarbeam on Flygon, particularly without Sunny Day anywhere on his team, strikes me as a… poor decision, especially when he could have just given it Flamethrower or Fire Blast.
Alder certainly gets more difficult by losing his focus on Bug-types, and there was nothing about Escavalier or Accelgor that made them strikingly appropriate for him (I mean, Reuniclus and Chandelure are kind of weird choices for Alder as well since they’re very calm and subdued by nature, but whatever). Conkeldurr and Braviary appear on his casual Black 2/White 2 team as replacements for Druddigon and Vanilluxe, and I think they’re good ones; they’re more dangerous on the whole, and they fit Alder’s energetic style. It’s unfortunate that Bouffalant is gone, because he almost works as a secondary signature Pokémon for Alder, sharing his excitable temperament and ridiculous hair. Krookodile is okay as a replacement, I suppose, and probably stronger. I’m neither here nor there on Alder’s lineup as a whole.