That's a TRICKY question.... I will simplify it for you
- Who was the MOST villainous character? Orochi 100%.... His actions were far more villainous than any other character in Wano
- WHo is the MOST IMPORTANT villain character in Wano? Kaido 100%... cause the arc is more centered around defeating him since he's more important to defeat than defeating Orochi.... in other words, defeating kaido, means you defeated Orochi (despite what happened last chapter between them and Kaido betraying him)... basically Orochi got his backup and strength from Kaido... Defeat Kaido, and you win this war, but defeat Orochi alone, it doesn't mean jack shit, cause kaido controls everything in the background...
- Who was the least villainous, but with IMMENSE influence for any side this character is on? Big Mom... she did the LEAST villain moves this arc, but she has the strength/influence to affect the course of this war.