The Irtysh is the tributary of the Ob often called the world’s longest tributary river.
The Irtysh begins as the Black Irtysh in the Mongolian Altai of Xinjiang, China. It flows through Lake Zaysan and then crosses Kazakhstan before entering Russia. Near Khanty-Mansiysk, it joins the Ob, creating the enormous Ob–Irtysh river system of western Siberia.
The Irtysh is about 4,248 kilometres long when measured from its distant source to its confluence with the Ob. That makes it longer than many major rivers that are themselves not tributaries. Its basin includes parts of China, Kazakhstan, Russia and Mongolia, and supports major cities such as Oskemen, Pavlodar and Omsk.
The Tom, Ishim and Tobol are important rivers in the same broad drainage network, but they are tributaries of the Ob or Irtysh rather than the answer to this question. In particular, the Tobol and Ishim flow into the Irtysh, making them lower-order tributaries within the larger system.