What is the system professional Pokemon players use to choose the best natures for their Pokemon?

Question

and how do they know if its a bad nature ?

Answer 1

Each Nature, increases a stat and decreases a stat. There are some natures which are neutral (don't do anything)
Players can check what natures do by putting them on the Pokemon Showdown teambuilder.

[Here you can see what each nature does][1]
[1]:http://bulbapedia.bulbagarden.net/wiki/Nature

As for your other questions... That depends on what the players strategy is, for example: Mega Charizard Y is Special Attacker and you give him Adamant which boosts Attack and decreases Special Attack, that would be useless.

So overall there is no particular "system" professional players use/might use.

To expand slightly, players will decide which nature benefits them the most. A sweeper will have to decide between a speed nature or an attacking nature. If the Pokemon doesn't outspeed many extra things with a speed nature, you choose power. If the Pokemon doesn't need the power boost, you choose speed. It's also reliant on the Pokemon's checks and counters. Charizard X's counters are usually speed based, trying to revenge kill it, so having extra speed really helps.
~AreWeFreshYet?

Comments

To expand slightly, players will decide which nature benefits them the most. A sweeper will have to decide between a speed nature or an attacking nature. If the pokemon doesn't outspeed many extra things with a speed nature, you choose power. If the pokemon doesn't need the power boost, you choose speed. It's also reliant on the pokemon's checks and counters. Charizard X's counters are usually speed based, trying to revenge kill it, so having extra speed really helps.

Answer 2

Hey there, SandDragon! Interesting question :)

Like Keromatsu correctly said, there is no professional way of selecting natures, nor is there a system to do so. It all depends on your instinct and your views on the given Pokemon. For example:

Azumarill @ Choice Band
Ability: Huge Power
EVs: 172 HP / 252 Atk / 84 Spe
Adamant Nature
- Play Rough
- Waterfall
- Aqua Jet
- Superpower

Take a look at this set. Notice that I've used the Adamant Nature. Can you guess why? Well, I used it because everything I gave to Azumarill helps it attack physically. The Adamant Nature increases Attack (which is useful), and decreases Sp.Atk (which we don't need anyway).

Therefore, professional players look at a) the Pokemon's set and b) the Pokemon's base stat, before choosing the nature.

Comments

I'm going to copy/paste my earlier comment here too since both of you missed the details :P

To expand slightly, players will decide which nature benefits them the most. A sweeper will have to decide between a speed nature or an attacking nature. If the pokemon doesn't outspeed many extra things with a speed nature, you choose power. If the pokemon doesn't need the power boost, you choose speed. It's also reliant on the pokemon's checks and counters. Charizard X's counters are usually speed based, trying to revenge kill it, so having extra speed really helps.
commented 16 hours ago by AreWeFreshYet?