I'm going with Sakazuki and then Blackbeard. While Sakazuki will be a big deal to Luffy in the final war, Blackbeard will definitely rise up to be the bigger threat. It's in his initial concept. Oda had intended Blackbeard to be stronger than all three Admirals put together until he ditched that power difference, meaning he at least wrote Teach as the grander antagonist in mind.
Keep in mind this isn't a "who will be the final villain" thread. You can still think Imu comes after both.
Keep in mind this isn't a "who will be the final villain" thread. You can still think Imu comes after both.

