Whether you are a DM or a Player, what do you consider the basic necessary materials that a Player must have/bring with them at a D&D game session? The bare minimum to be considered properly/politely prepared to play the game.
This is not a good complaint, because it is totally nullified by reading the first of two sentences in the OP: "a Player must have/bring with them at a D&D game session"This is not a good poll, because the answer totally changes depending on the game.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.