Held items aren't all that important in Gen III due to being generally inaccessible and underpowered. Here are a few good picks for Crobat, however.
Leftovers / Shell Bell - They're better reserved for team members with good bulk, but these are some of the best for playthroughs. In RSE, the Sitrus Berry is set to heal 30 HP, and berries like Aguav and Figy heal as much as Leftovers do in two turns, making them ineffective.
Lum Berry - Avoiding status conditions like Paralysis, Sleep, and Burn are great for Crobat. It and other status-healing berries might be better to reserve for fights you'd know you'd need it for. Otherwise, they'll be wasted on unnecessary trainer battles.
King's Rock - Effectively a 10% chance to skip the enemy's turn upon attacking, beyond edge cases like Inner Focus, as Crobat will outspeed just about everything the game throws at you, so long as it's decently leveled. It's only gotten right before the seventh gym, however.
Sharp Beak / Spell Tag / Silk Scarf - Boosts the main moves Crobat will be using for most of the game by a mere 10%. It doesn't need Normal coverage like Return as much as other Pokémon, but the Silk Scarf is easily accessible early into the game, while the others are a 5% chance from certain wild Pokémon. You might unintentionally get them, but don't deliberately go looking for the Sharp Beak or Spell Tag. It's not worth your time.
Hope I helped!
