The tree is Nebuchadnezzar. Daniel makes this clear to the king in Daniel 4:20–22. This dream shows that Nebuchadnezzar has tremendous earthly power. This power is very great politically, militarily, and culturally. It is even so great that people start looking to Nebuchadnezzar as someone who can provide them with happiness and eternal life. See also Daniel 4:11.
20 The tree you saw, which grew and became strong, so that its top reached to heaven, and it was visible to the end of the whole earth,