I got in an argument with a buddy of mine over this, and I'm interested to see what y'all think! What I've always liked about the Avengers is the diversity in their rogues gallery, and so my question is:
Who is the definitive Avengers villain, or villainous organization? When he/she/they show up, you're like: "Uh-oh!" In other words, if Marvel came to you and said, "Make the Avengers movie," who would you pick to be the main threat?
I haven't come to a conclusion yet, but right now for me it's a tie between Kang and Ultron, followed very closely by Thanos and then by Zemo and his Masters of Evil.
I tried to include as many options as possible (some are more of "neutral" or "sometimes-evil" characters), but if your vote isn't included then discuss! Have fun!