Veggies are a source of various phytochemicals (phyto- meaning plant, and chemicals referring to the fact that they are molecules). Some are naturally occurring in almost all plants, while others are occurring in only some families of plants.
These compounds are sometimes called phytonutrients, phytoalexins, or phytochemicals amongst other things. Each definition differs slightly, but they tend to refer to the same compounds.
Phytonutrients found in a wide range of plants in varying degrees are:
Pyrroloquinoline quinone is found in high levels in teas and some spices.
Green Tea Catechins, or the epicatechin family, tends to belong to anything derived from the plant Camellia Sinensis. This includes green tea, white tea, black tea, and oolong tea.
Isothiocyanates, including Diindolylmethane, are found mostly in Brassica vegetables like broccoli, cauliflower, or kale.
Brassinosteroids are also found in the Brassica family, and in highest levels in mustard.
Although there are no panaceas or cure-all tricks in nutrition, vegetables come close. That being said, they alone cannot make the best diet.
Note that there are healthy compounds found primarily in meat that cannot be obtained from vegetables.