Sometimes, villains are very simplisitc, but they're very effective in their simplicity. For Batman Beyond, that would be Mad Stan.
An anti-government nut who believes the only way to solve things is to "blow it all up!", he's tackled with Batman time and time again. Sometimes intentionally, sometimes not. Stan is strong, and durable, much to Batman's surprise, and his use of explosive is actually quite sophisticated, making him all the more dangerous of a threat.