What veg are you wanting to keep pests off GJ?
Calendula is the English, or Pot, Marigold. It attracts blackfly and their predator, hoverflies (which eat aphids). So they aren't a pest deterrent but they encourage hoverflies into your garden, which will help to keep the numbers of black fly in check.
Marigolds: there are many, including Mexican, French, African. French and African are quickly devoured by slugs and snails.
They are usually a companion plant for potatoes, the reason being either that the bugs that like potatoes hate marigolds, or else the bugs that like potatoes like marigolds even more (sacrificial crop.) and aim their destruction in that direction instead. But there's a real mix of views on whether it's effective or not.
French marigolds smell strong and as I understand it they help to repel or confuse pests that find our veg by smell e.g. cabbage whites, carrot fly, onion fly all find food with their noses.
Mexican marigolds apparently secrete something from their roots that deters or interferes with other plants - good if they are weeds, not so good if they are your precious veg.
I also read somewhere that marigolds badly affect the nematodes in the soil, but i am not 100% on this so will have to look into it.
I hope other members will correct me if they know different.