5 - Iridium
As well as sounding like something from Star Trek, Iridium is also among the rarest of all elements on earth. It's a shiny and very dense metal (the second densest of all elements, in fact), is resistant to nearly all corrosion, and is found most commonly in meteorites, which means the tiny amounts in Earth's crust likely arrived from the depths of the cosmos. Time to launch a mining expedition?
4 - Chlorine
It's a bit a contradiction, this stuff. It's found in abundance here on Earth, both in the oceans as well as in the crust in the form of ionic chloride compounds, and is essential for all life, but it's also highly toxic. The yellowish-green gas is a common component of chemical weapons and can cause major respiratory problems and death if inhaled. Sadly, it's still used in this way today, too...
3 - Plutonium
This one makes the list by notoriety alone. It's not actually the most radioactive element of all - that would be the ones us idiotic humans have created ourselves, naturally - but it's still incredibly dangerous, giving off alpha, beta and gamma radiation, all of which are deadly. It's rather concerning, then, that there is tons of the stuff in our atmosphere as a result of nuclear weapons tests...
2 - Mercury
Very well known as the only metal to take liquid form at room temperature, Mercury is also now just as well known for being extremely toxic. Obviously swallowing any liquid metal wouldn't exactly be beneficial, but Mercury can even be dangerous to the touch as it is absorbed through our skin. Less than a gram is enough to put someone down, but the effects can be very unpleasant even if you don't die...
1 - Hydrogen
The simplest and lightest element of them all, and one that's essential for... just about everything in existence! Around 90% of the observable universe is made up of hydrogen, for example, including the bulk of all stars. It is also responsible for something of more immediate importance to us too - water, not to mentioned tons of other stuff we couldn't survive without. Thanks hydrogen :)