"What vegetable is the healthiest?"
You cannot say 个 because it does not apply to all types of vegetable. You can say 哪种蔬菜/Which type of vegetable.
Leafy vegetables are counted by 棵, such as salad, lettuce, cabbage.
Round, oval vegetables such as pumpkin, potato, onion are counted by 个, small ones (e.g. cherry tomatoes, radish) can be counted in 颗 .
Relatively long vegetables are counted in 根 , such as carrot, a branch of celery, etc.