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Yes, they fought together, but the statement made who would win as individuals. No teaming up allowed. That's what makes it more interesting to figure out. Tee hee!

Of course they can beat each other. It's a matter of technique and using your surrounding.

Didn't anyone ever tell you? Joker's wild!

And very unpredictable. I'm sure he'd put up a better fight than just blowing some gas in the guy's face.

Being crazy and insane makes him fun! rofl

Sentry was based on Supes, they hardly differ in powers so I have no idea.

I don't really know Sentry that well.

Joker may be crazy, but Violator is sadistic, and only lives to see his victims suffer before killing them off.

Here's the weigh-in, sponsored by Wiki:
The Joker: The Joker commits crimes with countless "comedic" weapons (such as razor-sharp playing cards, acid-spewing flowers, cyanide pies, and lethally electric joy buzzers) and Joker venom, a deadly poison that infects his victims with a ghoulish rictus grin as they die while laughing uncontrollably. This venom comes in many forms, from gas to darts to liquid poison, and has been his primary calling card from his first appearance till the present. The Joker is immune to his venom.

The Violator: He is a being built of necroplasm, ego and lies. His fighting is vicious, born of a dog-eat-dog Hell. The Violator is a contradiction both in form and substance. For eons he has been a poisonous, dripping-fanged, spindly, terrifying, spiked demon with a weakness for the taste of human hearts.

Seeing that the Joker is human and Violator is necroplasm, Violator wins.