Items

Any item that boosts the power of you pokemons moves are good. For Example Silk Scarf for normal type moves. Also things like Kings Rock which can cause you opponents Pokemon to flinch.

Cleanse Tag
Amulet Coin (I forget the name, but it increases money earned)
Mystic Water (water pokemon)
Miracle Seed (grass pokemon)
Charcoal (fire pokemon)
Magnet (electric pokemon)
Twisted Spoon (psychich pokemon)
Any other item which raises power (black glasses)
Oran Berry
Sitrus Berry
Any other berry that will heal or restore

Leftovers would be my number one item to make a Pokemon hold

You could give your Pokemon stuff that restores it's HP like oran berries.
Or stuff that would boost the power of your pokemons moves. Like if you had a Blastois give it mystic water.

I always used items that boosted their attack Strength like charcoal for fire pokemon. But leftovers if also a really good one when battleing touph opponents.

Leftovers would be my number one item I always used items that boosted their attack Strength like mystic water for water pokemon

I whould pick qucik qlaw leftovers say if you have vensour mircle seed charzard charcoal blasoise mystick water magmite magnet twisted spoon phsik and resoreing items