Pokemon has long since been advertised as a world all about battling. All of the games focus on the combat aspect of it as does the anime. Each game has varying levels of difficulty but all of them can be enjoyed by casual or hardcore fans. With the introduction of battles with other people followed by how deep the mechanics can be, a competitive scene was created.
Competitive Pokemon is an entirely different ballpark than the casual battles people do with their friends. They use many Pokemon, moves, and items in ways some people wouldn't even imagine. Items are a critical component to being successful in the competitive scene, plenty of them making or breaking a good team.
10 Damp/Smooth/Icy/Heat Rocks
In Pokemon, there are several moves and abilities that bring about a certain harsh weather condition. These weather conditions are rain, sandstorm, hailstorm, and harsh sunlight. Each rock extends the duration of the weather condition it's associated with for three additional turns. Damp extends rain, Smooth extends sandstorm, Icy extends hailstorm, and Heat extends harsh sunlight.
Weather can play a big part in battles. There are many moves and abilities that adjust according to weather effects, some making the Pokemon faster or outright stronger. When paired with teams themed around weather, these items are a must.
9 Light Clay
Light Clay is a held item that goes on a Pokemon in charge of putting up Reflect, Light Screen, or Aurora Veil. Reflect halves physical damage when in effect while Light Screen halves special damage. Aurora Veil does the job of both but can only be active in a hailstorm. Damage reductions are lowered to only reducing a third of the damage in double battles or higher.
This item is extremely important when it comes to sustainability in competitive battles. Longer-standing barriers allow Pokemon to survive attacks that would have KO'd them and generally take less damage. This countered by the fact barrier-breaking moves and abilities exist, rendering this item useless if one is used.
8 Eviolite
The Eviolite is an item that only has a place on Pokemon that aren't fully evolved. If a trainer decides to hold their evolution back for this purpose, Eviolite rewards them with a 50% boost to both defensive stats.
While it may not seem like a worthwhile trade-off to most, it has sound logic in most examples. Evolution can grant an extraordinary boost to stats and expand their move pool, so why use Eviolite? For Pokemon like Dusclops or Chansey, some can actually excel at their job of soaking up damage better than their final stage can with the Eviolite.
7 Focus Sash
The Focus Sash is a simple item, basically giving Pokemon the equivalent to the Sturdy ability. This means that if they take a hit that would've KO'd them and they were at full HP, they'll remain standing at 1 HP.
This item finds its place mostly on speedy, frail Pokemon that can rarely take more than one hit. A party member holding the focus sash gets an additional turn of action than it would have without it. One turn in the scope of a competitive battle can change the tides drastically.
6 Super-Effective Damage Reducing Berries
It's common knowledge that each type in the world of Pokemon has its own weakness. When struck with a move it's weak against, a Pokemon takes double the damage it normally would have. There are a plethora of berries out there (Occa, Passho, Rindo, etc.) that can be held to half damage taken from its respective super-effective type.
Holding a berry that reduces inflicted super effective damage is great to have. Pokemon holding one of these have a much better chance of surviving if struck with their weakness. It serves a similar role to the Focus Sash as it also helps Pokemon stay on the field longer. This only matters if the Pokemon in question has the defenses to take the move's base damage in the first place, however. If not a Focus Sash is better to use.
5 Assault Vest
The Assault Vest is an item made for Pokemon naturally gifted in special defense. When held it disables the use of status moves, ones that don't directly deal damage and instead have an effect. It grants the holder a 50% boost to their special defense in return.
This is an item that can be used to give a Pokemon with decent special defense better odds of survival against special attackers. On Pokemon like Espeon or Gardevoir, it can provide them with more special bulk while they go on the offensive with their own high special attack.
4 Leftovers
The Leftovers is designed for Pokemon with the intention to stay in a battle for a long while. At the end of each turn, the holder of this item regenerates a 1/16th of their maximum HP, which may not sound like a lot but it isn't taking into account other factors.
A Pokemon's natural bulk and stat-boosting moves could very well make it so the health restoration undoes a decent number of the damage it just took. It can also be used with moves like Substitute and Protect which give it cover while it heals up. Paired with Toxic and another move to help sustain them, the Leftovers are a big player in stall teams and many other strategies.
3 Weakness Policy
The Weakness Policy is an item that requires a hit from a super-effective move to activate. While that alone will turn away many, the payoff is well worth it if they survive. As a reward, the Pokemon in question is granted a 100% boost to both attack stats, doubling their base number in each.
A natural challenge with using this is finding a way to reap the benefits while taking as little damage as possible. If a strategy can be made in double battles to use it or the holder is able to withstand one attack, their offensive ability makes a drastic jump. It could even be timed with the use of a move like Agility to greatly boost their speed, giving them the potential to be a team sweeper.
2 Choice Band/Specs/Scarf
Choice items provide a great boost to one of three stats, all of which serve a purpose for offensive pressure. While the Choice Band boosts attack, the Choice Specs boost special attack and the Choice Scarf boosts speed. Each is a 50% increase, but that Pokemon is locked into the first move they use until withdrawn.
A choice item is usually put on heavily offensive Pokemon with one powerful move that can clean house like a Dracovish with Fishious Rend. Said choice item is usually picked to make up for the aspect the Pokemon is lacking in, such as giving more power to a speedy Pokemon and vice versa.
1 Life Orb
Life Orb is an extremely popular item, arguably being one of the best for competitive battles. It boosts the output of the holder's damaging moves by roughly 30% and reduces their HP by 10% of their maximum each time they attack. Life Orb is highly versatile because it lacks the move restriction choice items have.
While the health drain is a cost to keep in mind, it's a fairly minor one. A purely offensive Pokemon likely isn't going to be on the field for very long unless they're cleaning out the enemy team. This item can be used on all manners of offensive moves, physical or special. It can easily make the difference between needing to hit something twice to KO them or just a single attack to take them out of the game.