Source: wikipedia.com

5. The Sentry

(The Sentry/New Avengers/Dark Avengers, Marvel Comics)

Aw, God, why? Why am I doing this to myself? Why am I taking time out of my day to think about The Sentry? The sun is shining, the bees are buzzing, but I'm sitting here on the couch, telling you about Bob Reynolds.

Bob was a meth addict searching for a fix when he broke into an abandoned lab and stole a prototype for a serum a thousand times more powerful than the Super Soldier Serum. He gained powers like those of a million exploding suns and became The Sentry. Bob slowly forgot his drug addict thief origins, and began to misremember himself as being a mild-mannered man who suffered an accident and gained incredible powers.

Oh, and as it turned out, The Sentry's repressed evil side manifested itself as The Void, an evil darkness creature made of crazy. After some more forgetting and re-remembering, The Sentry joined the New Avengers, fought in the Civil War, and spent six issues moping about while the Hulk wrecked New York City, until he finally got off his ass and did something about it.

The Sentry was so boring and hated that for a long time, there was a literal fan bounty on his head. There was also the long-running (but now defunct) killthesentry.com. When a character's so bad that fans are literally willing to put up money to get rid of him, that's how you know it's a bad character.