A big, grey alien with several bone-like thorns all over his body.
A simple yet somehow over designed look that marks the spirit of the 90's in comics, Doomsday was the one who "killed" Superman on the iconic story that was pushed as a big deal at the time by DC Comics.

But let me tell you about Doomsday.

This character killed Superman in probably the most silly manner possible to a character of that magnitude: By punching him in the nose really hard.

But of course, despite that, the idea of the character is actually really interesting.
An ancient alien type from Krypton that got experienced over and over again, regenerating and evolving into a "perfect" being that could survive pretty much anything, while being conditioned to hate, target and kill Superman. In many ways, the comic book Doomsday is the exact concept the authors had for the character role in the story: A Superman killer, nothing else.

The "March towards Krypton" had the character walk towards Metropolis, destroying anything on his path including the at the time current Justice League, buildings, cars, really without a care. And that is a character defining moment. Doomsday imminent Superman fight was such a big deal that not even an entire group of Super Heroes or even Geography could stop his march.

The character evolved when he was eventually brought back, he was even the star of one of DC Comics last events before rebooting their entire comic book line with their "Nu-52" approach.

He even become an alien virus that targeted Kryptonians and changed their body to look like the monster associated with "Doomsday".

Despite all of that, in my mind, whenever I see the word "Doomsday" I relive the panels in which this hulking beast is marching towards it's destination, destroying anything or anyone daring to cross his path.