For general content:
- Enigma Berry - Very good generic defensive item if you want to survive while first ordering
- Luck and Odd incense - My favorite generic attack and defense incenses, they are very good if your stats are at the right place
- Tamato Berry - Sometimes you just need to be fast to finish someone off.
- Leftovers - Still a good item to slap down if you don't really know what you need.
- Life Orb - The power level of unconditional damage items has gone way down, but life orb is the easiest one to use when you need to hit things harder.
- Heavy Duty Boots - Good answer to rocks, good in a surprising amount of other cases. For example you can equip it if you are poisoned, or if you want to type "Steel Beam".
More specific things:
- Pomeg Berry is very good on anything with 80 hp or lower. it is an enigma berry that always goes off and cannot be knocked.
- Flame/Toxic Orb - I like trying to trick flame or toxic orb to opponents since it bricks their item slot and inflicts a condition. Any steel or poison mon with trick can use toxic orb, any fire mon can use flame.
- Shell Bell - Really high numbers on the item if your mon has metal sound or screech.
- Normal Gem - This is actually my favorite damage item at level 4, since you can 30 damage hyper beam people for a big burst.
- Kelpsy Berry - Funny on fake out mon who are not primarily physical such as gothitelle or munkidori.
- Barometer - Really good on any sandstorm storm mon and possibly other weather as well.