It's a tough letter to choose just one, but here's my pick!
Pete Stanchek (Sting)
You know, it's a sad thing in comics when the sharp downfall of Pete Stanchek's character doesn't even stand out that much compared to similar situations with other comic book characters.
Created by Jim Shooter, Pete was the character that was the driving force behind Harbinger (and yes, Sting is a dumb name), the only person who could challenge Toyo Harada, power-wise.
There is this great bit where his girlfriend (Kris) was only dating him because he was subtly controlling her mind. Of course, he was not a jerk, so he stopped doing it - but it is still pretty messed up, and I like how it weighs on Pete.
Then, we get to the end, and we learn that this whole thing was a ploy by Harada, who was manipulating the people around Pete to get to this point.
This leads to an epic battle royale between Pete and Harada, which ends with Pete powerless and Harada comatose.
Then the lameness begins.
Pete is then brainwashed by a recovered Harada into becoming "The Harbinger" (although the Harbinger's identity is secret at first)
Meanwhile, the mysterious Visitor shows up, and it is just blatantly Peter. There really wasn't much of a secret there - until they reveal - AHA! - Pete is the Harbinger, not the Visitor! Which made no sense, so of course, shockingly, the Visitor IS Pete, but it's Pete from an alternate future!
He teams up with the now-free-from-brainwashing Pete and they help save the world.
Such a cool character at first and such lameness at the end.