The Dark Masters
When the Digidestined return to the Digital World after defeating VenomMyotismon and picking up Kari, they find that decades have passed and the Digital World is now ruled by four evil Mega level Digimon, known as the Dark Masters. (Nice surprise, huh?) The Dark Masters have basically been tyrannizing all the Digimon, and it's up to the Digidestined to take care of them.
MetalSeadramon is the extremely friendly *sarcasm* controller of the DigiWorld's seas. He's the first Dark Master the kids meet. After Izzy finds out that WarGreymon is a "Dramon Destroyer" (i.e. he's good against "dramon" Digimon), they go defeat MetalSeadramon, but unfortunately they're not in time to save Whamon, who gets reconfigured.
Puppetmon just wants to play - unfortunately, it's not what you'd usually consider "play". Generally, his idea of play constitutes blowing up other Digimon, which isn't well-received by his "playmates" Deramon and Floramon.
Machinedramon is in charge of (well, read "dictator of") the Spiral Mountain cities. He controls a big army of robot Digimon who he sends after Izzy and Tai, when they're hiding out and looking for Kari's medicine. Luckily, WarGreymon saves the day and gets rid of the robotic creep.
Look at it this way, Piedmon may be a clown but you wouldn't want him at your birthday party. He's the last and strongest of the Dark Masters, and it pretty much takes all the Digidestined's power to stop him. He turns everyone except Kari, T.K. and Patamon into a keyring. Angemon manages to digivolve to MagnaAngemon - the last one to digivolve to Ultimate level - and restore the others so WarGreymon and MetalGarurumon can send him through MagnaAngemon's Gate of Destiny.