Strongest Villian in my opinion is Malomyotismon or whatever. They couldn't really actually kill him without doing the:
"I wanna be a doctor!"
*Arm explodes* "Noooo this doesn't make any sense!"
Thing.
But seriously, he feeds off of anything dark in a person's heart. That's pretty strong.
The strongest good guy is Gatomon. She can maintain champion form for however long she wants. ((Probably due to torture and whipping >>))
Anyway yeah. I watched the first, second, and third series. The third was actually good during the second half of it.