So many great replies already, but to me, there can be only one because it directly affects the comics of the present in a way that Rob Liefeld's art doesn't.

That's easily GOT to be Amalgam comics, right?

The art of Rob Liefeld is basically the reason I stopped reading Marvel Comics, or really any non-indie comics, until a friend lent me a bunch of Sandman trade paperbacks.

Say what you will about Valient, but I actually dug a lot of the early issues of their stuff. Harbinger was a pretty decent read for a while.

I liked Valiant back then, it was something different from Marvel & DC but still similar. I still have a Shadow-man t-shirt somewhere.

The normative of the antihero archetype. Yes, antiheroes existed before the 90s in comics and other media, but there's only so much that can be explored about this type of character. I believe the propagation of antiheroes in the 90s has created a kind of go-to trope for lazy writers (and readers), i.e. - a character…

We call that Liefeld-ism and it's a serious disease.

Armageddon 2001

Yay! An opportunity to post one of my favourite snarky things of all time.

If you can think of a better way to organize all 11 flavors of Tic Tacs and all 34 flavors of Trident gum, I'd like to hear it.

Even on a real picture his hands are not showing!

So many good candidates here. This is, I think, the 90'sest character ever. Hyperstorm, the son of Franklin Richards and Rachel Grey- a villain from the future with vaguely defined "energy manipulation" powers. Plus, he has shoulder pads, a weird mask, and long hair just like every 90's supervillain.

Fantastic Four circa 1993

It's low hanging fruit I know, but holy god, this rendering of Captain America just boggles the mind.

Multiple variant covers. Definitely multiple variant covers; that alone might have contributed most to the eventual and never-to-fully-recover-from crash of the industry because of over-zealous collectors and over-eager marketing decisions. Greed: Fucking up America since July 4th, 1776.

You said The Artwork of Rob Liefeld, which pretty much covers it: No feet. Such Pouches. Wow Waist Line.

Polybags. Fucking polybags.

HOLY SHIT, THE LEVELS OF AWESOME CAN'T EVEN BE DESCRIBED! :D

!!! You included my Age of X group!! You just made our freaking DAY. THANK YOU THANK YOU THANK YOU.

I think it's great when Tom Cruise can just put on a mask and mingle freely on the floor at Comic-Con.