Jupiter, Mars, Venus and Saturn have been visible to the naked eye as long as mankind can remember.
Venus
Mars
Venus.
Venus
well the moon... but that's not really a planet so logically we found the closest planets first.
The Babylonians, who were credited with writing about both the element and the planet about 3500BC but this does not mean they were the first to observe.
galileo, he found it with a telescope.
First and recently are mutually exclusive and cannot be answered
There is no such planet
Uranus is the answer, discovered by William Herschel in England.
Mars has a ravine which is the first ravine found on a different planet.
Mercury has been known since ancient times. No recorded person first found it, and it was known to be a single planet (rather than one you see sometimes at night, and another you see sometimes in the morning) about the first century BC.