Everything that makes a mark. Graphite, colour, pastel, charcoal, ink and marker, plus the erasers, sharpeners, clips and fixative that go with them.
Six routes by medium.

Sketching, tonal work, life drawing
Graded graphite from 2H to 10B, plus clutch pencils, mechanical pencils and woodless sticks. Koh-I-Noor, Derwent and Bruynzeel.

Permanent and water-soluble
Sets from Caran d'Ache, Koh-I-Noor and Pentel. Water-soluble pencils give you a drawing and a wash out of one tool, which is handy when you're travelling light.

Soft, oil, willow, compressed
Willow and compressed charcoal, soft pastel sets, oil pastels and Conté crayons. Blending stumps and fixative to keep the work where you put it.

Line work, technical, illustration
Pigment fineliners from 0.05mm up, technical drawing pens and gel pens. Pentel Pointliner, Edding and Copic Multiliner.

POSCA, on almost anything
Water-based pigment paint markers that write on paper, wood, glass, fabric, stone and even trainers. Extra fine through to broad chisel.

Calligraphy, wash, sumi-e
Indian ink from 120ml up to five litres, Chinese sumi ink, coloured drawing inks, and bamboo dip pens with nibs.
The same range, sorted how you prefer.
Making sense of graphite grades. H is hard and light, B is black and soft, HB sits in the middle. For tonal drawing you want a range: 2H for construction lines, HB and 2B for most of the work, and 6B and above for the darks. A set of six graded pencils will do more for your drawing than one expensive one.
Charcoal isn't one material. Willow is light, dusty and forgiving, and it brushes off, which is the whole point of it. Compressed charcoal is dense, much darker, and it commits. Most people end up wanting both, plus a putty rubber that lifts rather than smears.
Pigment ink or dye ink. If the work needs to last, or you plan to wash over it, look for pigment rather than dye, since dye-based ink bleeds and fades. Nib sizes from 0.05mm to 0.8mm cover most illustration work.
Looking for versatility? Paint markers are the most adaptable thing on this page. POSCA is water-based pigment in a valve pen, so it goes opaque on dark surfaces and works on paper, wood, glass, fabric, metal and plastic. That's why they turn up in primary schools and on trainers with equal frequency.
Don't skip the erasers. A putty rubber lifts graphite and charcoal without abrading the paper. A plastic eraser removes cleanly. An eraser pencil gets into detail. They cost very little and they change how a drawing looks more than most people expect.