To create an effective Pokemon team, players must take many things into consideration. Two of those considerations are the Items that a particular Pokemon can hold and how effective the Item can be. Held Items were introduced into the series in Gen 2 and, with the exception of Gen 7, new items have been added in each generation since.
An Item is an object which the player can pick up, purchase, or otherwise acquire during exploration. Items can then be given to Pokemon, and will have varying effects depending on the Item. Possible effects include boosting the power of a specific type, increasing a stat, or healing. With that in mind, let's take a look at the strongest Pokemon/Item combinations that players can put together for each generation that included this mechanic.
6 Gen 2: Marowak With A Thick Club
Marowak, on its own, is a solid Pokemon. Even without the help of any Item, it has good Attack and Special Defense with high Defense, great ability options, and good typing. It even has a great regional form. That already solid Attack stat, however, can be taken to much greater heights with the Thick Club Item.
The Thick Club, simply put, doubles Marowak's current attack stat. Players will immediately recognize just how wonderful an improvement that is. Thick Club takes this Pokemon's decent-at-best offensive potential and boosts it greatly. Considering both forms have access to strong attacking types (Ground and Ghost, in its Johtonian and Alolan form respectively), all of this combines to make Marowak an extremely hard hitter.
5 Gen 3: Clamperl With A Deep Sea Tooth/Scale
Similar to the previous entry, the Deep Sea Tooth doubles the current Special Attack stat of the holder, while the Deep Sea Scale doubles its Special Defense. It is up to the player which one they want to take. Clamperl is a rather forgettable Pokemon without one of these items, as it doesn't really have a lot going for it until it evolves.
These items, however, actually make a huge difference with this particular Pokemon, which has good typing and access to Ice moves. Since the majority of players lean toward offense over defense, the tooth is the better option than the scale. Remember, though, that increased defenses also has its advantages.
4 Gen 4: Togekiss With A King's Rock
Togekiss is a stellar Pokemon, with great stats, great moves, great abilities, and more. It has access to the Move Air Slash, a strong Special Flying-type Attack that has a 30% chance to make the opponent flinch. When Togekiss combines this with the Ability Serene Grace, it gets boosted to 60%. If taken a step further with the King's Rock Item, it goes up to 70%.
This item effectively means that more than half of the time, if Togekiss hits another Pokemon with Air Slash under this combination, they most likely will not get a turn. This is useful on many levels, because it means their opponent has to jump through a ton of luck-based hoops to get any chance of moving.
3 Gen 5: Dusclops With An Eviolite
As many of its opponents have learned, Dusclops is exceptionally difficult to remove from battle. Dusclops has incredible defensive stats, both having 130 points at their base level. An Eviolite increasesthe defense stats of a Pokemon by 50% if it can still evolve.
Dusclops is the middle form of the Duskull line, meaning it can take advantage of this effect. Eviolite boosts Dusclops defenses to absurd levels, even more so when one considers that it can further boost its durability with moves like Calm Mind, Will-O-Wisp, or Rest. Finally, as a Ghost-type, it cannot be hit with Normal or Fighting-type moves. Pokemon don't get much more defensive than this.
2 Gen 6: Tyranitar With An Assault Vest
Tyrantitar is a beast of a Pokemon, which is usual for a pseudo-legendary. However, for any player that wants, even more, there is good news. The Assault Vest item was made for this Pokemon, as it boosts a Pokemon's Special Defense stat by 50%, at the cost of only being able to use damaging moves. Honestly, with how strong Tyranitar is, that "drawback" is hardly an inconvinence.
Tyranitar's Sand Stream ability takes this further, as it summons a Sandstorm whenever Tyranitar is sent in. Sandstorm boosts the Special Defense stat of all Pokemon in the battle currently by 50% again. This means that with the Assault Vest, Tyranitar's Special Defense is essentially doubled.
1 Gen 8: Toxtricity With A Throat Spray
Toxtricity was a lovely addition introduced in Gen 8. It's decently fast, with good HP. It also has a good attack stat and an even better Special Attack stat. Electric/Poison is an amazing type combination, as it only has 2 weaknesses, and 8 resistances. Punk Rock is also a great ability, boosting the power of sound-based moves by 30%, and it matches Toxtricity's aesthetic perfectly.
Speaking of which, the Throat Spray item increases a Pokemon Special Attack by 50% after they've used a sound-based move at least one time in the current battle. Since Toxtricity is going to be using a lot of those anywaay, why not get a power boost from it? It makes this already great all-around Pokemon even better.