On the periodic table, osmium is generally recognized as the densest element at standard conditions.
Osmium has atomic number 76 and symbol Os. Its measured density is about 22.59 grams per cubic centimeter at room temperature, slightly greater than that of iridium. The exact comparison can vary a little with crystallographic and measurement details, but osmium is the standard answer in reference tables.
Osmium is a hard, brittle, blue-gray transition metal in the platinum-group metals. It occurs naturally in platinum ores and in alloys with iridium. Because osmium compounds, especially osmium tetroxide, can be highly toxic and volatile, the element is handled cautiously.
A common error is to answer iridium, whose density is extremely close. Tungsten and platinum are also very dense engineering metals, but their standard densities are lower. Density is mass divided by volume, so this question is about compactness, not atomic number or atomic mass alone.