A shulker of useful items to be kept in the inventory at all times in order to make gameplay as smooth as possible. This reduces the need to access items in your ender chest as often, saving precious time. Explanations for rationale in building this utility box are below:

Ender Chest - Can't live without it.
Logs - Essential for crafting many items.
Coal - Compact fuel source, can smelt nearly 3 shulkers of items.
Iron - Essential for crafting many items.
Bone blocks - Useful for any kind of farming needs.
Wool - Stackable form of beds when combined with logs.
Obsidian - Making portals (redundant with echests, but saves time having to place and mine them).
Shulker shells - Making shulker boxes (when combined with logs).
Golden carrots - Spare food.
Fishing rod - For afk fishing, wherever you may be.
XP bottles - For reparing elytra or tools.
Flint and steel - Lighting portals, or whatever else.
Netherite Hoe - Silk touch, for mining blocks quickly.
Fortune III Netherite Pickaxe - I prefer to have my inventory pick to be silk touch, but sometimes you need to "break down" ores for use e.g. quartz.
Ender pearls - Generally useful, although can be removed.
Bow and arrow - For ranged attacks or lighting TNT from afar. I never use bows but it's nice to have handy.
Anvils - I find it handy to have these ready to go if I am making lots of gear.
Elytra - A spare you can switch to instead of using up XP bottles if they are in short supply.
Nametags - For preventing hostile mobs from despawning, or just for fun.
Torches - Spawnproofing new areas.
Ice - Can be used to create an infinite water source.
Lava bucket - For killing villagers, destroying items, etc.
Totem - A spare if you accidentally pop one (e.g. flying into the ground).

(May be edited later to fill the shulker box)