Best Main MCU Villain?

Who was the best main villain of the MCU films? I am not making Bucky a choice as he wasn't a main villain, he was a tool of Hydra and Alexander Pierce.

*Edit: I forgot to check the submit poll option, if a mod could make this into a poll I would appreciate it! :)
- Red Skull
- Iron Monger
- Whiplash
- The Abomination
- Loki
- Aldrich Killian
- Malekith
- Alexander Pierce
- Ronan the Accuser
- Ultron