Oak log, or as a geometric object, a solid cylinder.

A sphere only has one contact point with the ground, which means all it's weight is concentrated onto one point, which increases the pressure on the ground and the sphere, which could cause deformation, depending upon the ground and sphere. While a cylinder has it's weight better distributed along it's axis, reducing pressure.

Having more contact with the ground also reduces occurrences of slipping, which is usually undesirable for "perfect rolling." A solid cylinder does have a higher moment of inertia (assuming the sphere is the same mass and density), which means it takes more energy to get it rolling, which may be bad, but the higher energy also means it will roll longer than a sphere, since each will experience the same frictional force, but the cylinder will have more energy that friction has to take away before it stops rolling.

In short, there are reasons why tires are not spheres, but cylinders.