Metal-filled PLA, at 2.0 to 4.0 g/cm³ depending on how much metal powder the blend carries. Nothing else on this site comes close: the next densest are PET at 1.32 to 1.40 and ULTEM at 1.29 to 1.34.
At 3.2 g/cm³ a kilogram occupies just 312.5 cm³ and gives 129.92 m of 1.75 mm filament — well under half what plain PLA gives from the same spool.
The density is the product
Nobody buys metal-filled PLA for mechanical properties. Its tensile strength is 10 to 30 MPa, well below plain PLA, because the metal powder interrupts the polymer matrix rather than reinforcing it.
What you are buying is mass, appearance and finish: a part that feels like metal in the hand, takes a polish or a patina, and reads as bronze, copper or steel rather than as plastic. For props, jewellery masters, model weights and decorative work, that is exactly the specification.
The cost consequence is severe
At $45 to $100 a kilogram, and with more than three times the mass per cubic centimetre, the per-part cost is brutal. A cubic centimetre of metal-filled PLA at $70 a kilogram costs $0.22 against PLA's $0.027 — eight times, for the same shape.
A 12.1 cm³ Benchy hull is 38.7 g in metal-filled PLA — $2.71 of plastic against $0.33 in ordinary PLA.
It will destroy a brass nozzle
Metal powder is abrasive. A brass nozzle wears measurably within tens of hours on this material, and the wear is gradual: the orifice widens, extrusion goes over-wide, and dimensions drift before anything looks obviously wrong.
Fit a hardened steel nozzle before the spool goes in, not after the first batch measures badly.
The other dense options
PET at 1.32 to 1.40 g/cm³ and ULTEM at 1.29 to 1.34 are dense for engineering reasons rather than decorative ones. If you want mass specifically, metal-filled is the only material here designed to deliver it.
Glow-in-the-dark PLA is worth mentioning alongside it. Its band reaches 1.40 g/cm³ because of the strontium aluminate phosphor loaded into it, and that filler is abrasive too — a spool that looks like ordinary PLA and behaves like a filled one on both counts.
The length and weight converter opens at 3.2, which is a metal-filled grade rather than anything you would print a bracket in. Drop it to 1.40 and you have the glow-in-the-dark figure — the densest material most people will ever actually load.