I myself have no trouble with Darius, when I go top I hope to see a Darius because I never lose to them... But that is because the only top champ I play is a counter to him. My problem is that my friends always whine about how overpowered Darius is and that they can't do anything to counter him and they only win against him if the Darius is completely retarded etc etc. So, how should they counter Darius?

Teemo, Yorrick, Kayle, Jax, Mordekaiser, Kennen, Renekton.

Darius is very potent as a top-laner because he requires that you counter his ultimate and passive by building health, which hurts your damage output since you already have to build armor to counter the rest of his abilities.

Countering him is actually not hard. The key in a Darius counter is that you want someone who can either negate his damage or someone who can prevent him from doing said damage at all via either range or a CC.

Jax negates his damage quite well, and also has a hard CC which allows him to trade efficiently with Darius. As an added bonus, since Darius does not really become potent until level 6, when he can use his ultimate for true damage, Jax will be at little disadvantage in lane (Jax himself does not become potent until level 6 as well, so the lane will generally be more harass/farm and less killing). Since Jax becomes very strong by end-game, where Darius becomes mediocre damage-wise, Jax will not have a hard time winning lane even if Darius manages to pump strong damage mid-game.

Teemo is another good counterpick, though for a completely different reason. Teemo's blind prevents Darius from trading early-game, when Teemo is most squishy. By the time Darius can efficiently trade with his skills, Teemo will have already built some of his core on-hit tanky items to be able to trade. Also, Darius synergizes well with jungle ganks due to his pull (for initiating) but his need for CC (since he has almost none EXCEPT the pull), and Teemo's mushrooms allow him to avoid being ganked, thus severely gimping Darius' damage output and allowing him to win the lane.

You forgot Kayle ... EDIT: Oops, the one champ I skipped over happened to be Kayle. xD!

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How about playstyles that work to counter Darius? They don't like playing draft mode (Their only reason to play draft mode would be to ban Darius, which they don't like having to do). And uhh... Apparently, so they say, they don't want to have to always play a champ to counter a Darius that they assume is going to be on the enemy team.

I think you need to re-read my list.. It contains Kayle.

darius fails against ranged champions who can harass him from a safe distance.

I've had success against him using kennen, vlad and lee sin.
lee sin's quick in and out bursts whittle him down slowly while keeping lee safe

vlad and kennen have strong matchups because vlad has more sustain and trading with a smart kennen will always result in darius being behind. darius can't get in range and even if he has the balls to do so he'll get stunned in minions and get crapped on.

essentially you'll outfarm him and scale better into late game while he turns into a useless brusier thats easily kited

also, most darius players (not all) are cocky as hell and ganking them is easy since he doesnt have an escape.

May I add, that with Kennen and Vlad, you have an energy and manaless champion vs Darius,who requires mana. If he tries to harass a lot, he wastes mana. If he does nothing, you get "free" harass essentially.

As anyone, I'd say that he'll probably out-damage you, unless you're someone like Renekton with an amazing early game.
As a ranged person, obviously harass when you have the opportunity, but don't let him close to use Apprehend.
As a melee, you'll be risking health for last hits. In any case, I think that your best bet is to out sustain him, since he has none. It's hard to escape from Darius, because of his passive, his W, and that his Q does more damage in the outer ring, and Apprehend.
If he can out-harass you, get a chain vest to mitigate the damage. If you can out harass him, you shouldn't have a problem.
You'll see that the champions people recommend against him all have either sustain or better harass, through range or shields.
It may also help to list who your friends tend to play as, so that people can give you more specific advice.
And don't tower dive him unless you can easily get away with it. He can get you killed by the tower with his burst and slow, even if he dies. He may also have Summoner Heal.
Ofc, anyone can correct me if I'm wrong, I'm not an expert.

I personally like Kennen as a matchup. You harass with Qs, walk away when he tries to go in for the pull. When he does successfully land a pull, you lightning rush away.

Thank you very much for the tips people, I'll get them to take a look.

They play Renekton, Kennen, Fiora, Riven, uhh... I forgot the rest.