There are many standout moments, especially with the M3 in pre-TS. It’s surprising no one's posted Zoro’s "nothing happened" or Sanji’s "I needed a light" panels yet—those were gold.
This scene though is peak Luffy moment. Flags play a huge role in One Piece, so this panel also really carries the series’ identity.
Technically not a single panel but Law’s 2-page speech about Marineford and breaking the gears while cutting everything in half is probably my favorite moment in post-ts: