I’ve been lucky and have worked on missions to study Earth, Venus, and Mars. It’s hard to choose one particular thing, but maybe it was Magellan, the Venus radar mapper, which helped us discover more than 90% of the surface of Venus in better detail than ever before. And because there are no oceans on Venus, we know more about Venus’ surface than that of the Earth (though I am very glad we have oceans!)
I’ve discovered salted caramel spread that you can put on toast – that was pretty exciting! 😉
As an engineer I don’t really make discoveries myself – I design things, in this case a Mars rover, that will make the discoveries for itself once it is on Mars. It is due to launch in July this year and will get to Mars in March next year (it takes a long time to get to Mars) and then maybe it will make some discoveries.