Evil Is Cool

4 vs. Satan:
* Satan himself, as well as most of his helpers, since they give some awesome battles. And every climatic battle that has an awesome battle music.
Alex in Princess Quest:
* Malthor van Dracula, who pretty much signifies the The Chessmaster as an antagonist.
The Stone of Power:
* Artimus and Balthar, who transform into powerful monsters when you fight them.
The Three Kingdoms
* While the two kings are utter scum, they nontheless give the player an awesome fight.
Dragondawn
* The Three Dragons. They're powerful, they trash some of the heroes' allies and then give the heroes one seriously difficult battle.
Harter Knall
* The S6-unit. All of them make a good battle during the climax.
Hero Story
* Every major villain, since they're both hard to beat and repeatedly put the hero in check before finally being fought.
Kill the Demons
* Casper and Rustoc, as they're far more powerful than every boss prior to them.
Satan Saga
* Walter and Meynard. Sure, they're mass-murderers with no redeeming traits whatsoever, but damn do they give the heroes an awesome battle and they're somewhat successful as well.
* Trianuk. Although he has repugnant moments, he's just as much a challenge despite being a old man.
* Satan, who quite possibly is the most challenging boss in the game.
* King Greynor in one version, offs his men by himself before giving the heroes their final battle of the game.
Story of Hero:
* Most antagonists, as all of them are powerful and give the heroes at least one troublesome fight.
The Legend of Devils
* Satan and every major villain as all of them give the player a challenging battle. But Satan himself more than anyone else.
The Sidestory Duology:
* Lavius and Demonica in how they give they party some obstacles before going down.
* Sez, Soult and Ravydion. They give the group one challenging battle in a duology most lacking in those kinds of battles.
The Time Of Bloody Fear:
* Maltros and his people are surprisingly powerful, as the game doesn't have much magic users otherwise.