Diamond swords are the most common weapon in ExplodingTNT's videos. This is because they were the strongest weapon (which is outdated, as the diamond axe was made more powerful in 2016, and netherite items were introduced in 2020) in Minecraft, and thus the rarest in the game. They are usually used by pros for this reason, along with diamonds in general. They are also occasionally used by Noobs or other players for comedic effect or to generally attack something.
Netherite Sword[]
As of 2021, most diamond gear was replaced with netherite gear, as evidenced by Dream and Technoblade using netherite swords in If Dream Took Over Minecraft.