* Wish-Full Seed: This item can be used to increase the power of a move by 1.5x, which can be considered a "nice" effect for boosting your team's strength.
* Miracle Seed: This item can be used to revive a fainted Pokémon, which is definitely a nice thing to have in a pinch.
* Rainbow Seed: This item can be used to temporarily increase all stats of your team, which again, is a nice way to give your team an edge.
Ultimately, the "nicest" item depends on what you need in a given situation.
It's also important to note: There are no items in the game that directly grant wishes or allow you to "wish something nice". The items listed above are simply items that can be considered helpful or beneficial to your team.