Which superhero is the team leader of the Fantastic Four?

The team leader of the Fantastic Four is Mr. Fantastic, the superhero identity of Reed Richards.

Reed Richards is a brilliant scientist, engineer, and inventor who helped create the Fantastic Four. During an experimental spaceflight, cosmic radiation transformed Reed, Susan Storm, Johnny Storm, and Ben Grimm, giving them extraordinary abilities. Reed gained the power to stretch and reshape his body, while also becoming the group's chief scientific problem-solver.

Mr. Fantastic's leadership is based as much on his intellect and planning as on his powers. He designs much of the team's equipment, including their spacecraft and specialized uniforms, and frequently directs their missions from the Baxter Building. Susan Storm, the Invisible Woman, is often the team's emotional anchor and has led the group in Reed's absence, but Reed is the standard canonical answer to this question.

A common mix-up is choosing the Thing because Ben Grimm is a founding member and the team's powerhouse. The Human Torch and Invisible Woman are also original members, but neither is generally identified as the Fantastic Four's principal leader.