I'm going to assume the intent here was my favorite mundane item, and not things like longsword, chain shirt, longbow, horse, etc.
It really boils down to two iconic items found in most adventurer's packs - the oil flask and the 50' rope (with grapple hook, if allowed).
An oil flask could refill a lantern, be poured upon a green slime/troll, thrown as a weapon, lubricate a rusty lock/hinge etc, coat someone/something to make it slick (harder to walk/grapple), among other uses.
A rope, however, allows you to climb up/down various pits, cliffs, dropoffs and other obstacles, especially handy if you are not a sneaky type or monk. You can rope a party together, bind prisoners, set traps (snares and triplines), secure people and/or items to horses, wagons, ships, and dragons.
I have to go with the 50' rope. Now if I could just convince my DM to allow me to tie knots while falling, I would be set for life! (Sorry, Dino).