Blue beat Lance before and he could do it again.
The situation between Wallace and Steven has never really been explained, but i've always assumed that Steven CHOSE to step down and that as the 8th gym leader Wallace just took his place.
So, its basically between Blue, Steven, Cythia and Alder. (Since Red's not in this list)
Personally i've always thought that trainers who specialize in one type were weaker, just because if you train a pokemon thats strong against that type then you can easily plow through the team. Which is why I think Steven can't be considered.
So, out of Blue, Cynthia and Alder who do I think is the strongest? Honestly I don't really know. I haven't even faced Alder in my White version yet so i'm not sure of what he's really capable of, but Blue, Cynthia and Alder-It would be one of them.
I think Cynthia would win. She trains Pokemon of different types, and the fact that they all have Perfect IVs. All that aside, she has the best tactics, and in Black and White, she's a secret boss, and I think a bit stronger than Alder.
So let's say Cynthia travels to Kanto to research some local myths.She goes trough the Cerulean cave full of strong wild Pokemon.She manages to catch Mewtwo and she just has her Milotic on low HP from the battle with Mewtwo.
She exits the cave and is just about to get to Pokecenter and guess who appears?Blue Oak who has just caught Mew to beat the crap out of her!
In-game wise, Blue actually does beat Lance and Wallace does beat Steven (I assume?)
We all know Alder is a joke with his half bug team - hell, nobody voted for him.
So between Blue, Wallace, and Cynthia, I have to say Blue BARELY wins with Cynthia as a close second if for no other reason than that Cynthia ditched her Water immune-rock resist which would appear to be very useful against Gyarados
Honestly, I don't like Cynthia, don't understand why everyone loves her, and also found her the easiest Champion (and who always gets hax for some reason).
I really like Lance, but his team in some versions is a bit of a joke. As are some of Wallace's Pokemon too. Really, Luvdisc? However Wallace's team otherwise is pretty good.
Alder has the three Bug types, but I won't judge him as I haven't (been stuffed) to verse him yet.
In the end I find Steven hard, as he uses very defensive minded strategy's that annoy the player, whilst also having significant offence (Metagross, Armaldo, Aggron). I also think that Blue is good, as he has good diversity and is very smart type wise.
In the end it would have to be either Steven or Blue for me, but I'm not entirely sure which one.