The answer is the neutral, or both.
The service enclosure must be bonded to the neutral. See above. A lot of service equipment, but not all, comes with the neutral bonded to the enclosure by the factory. If it comes bonded by the factory it is typically allowed to be used only as service equipment.
If there is a separate equipment grounding bar, it must be bonded to the service enclosure or the neutral. Since those two options are already required to be bonded, you'll be bonding all three together regardless of how you go about it. You are not required to have an equipment grounding terminal bar in service equipment, although it's good practice. (It is not a code violation to bond all the equipment grounding to the service enclosure with separate terminations, provided they are proper terminations. It's just sloppy when you could use a bar.) Usually equipment grounding bars are attached to enclosures such that they are bonded by the attachment. You can also use an equipment grounding bar as the means to bond an isolated neutral and the enclosure together.
The only grounding item that you may not bond to the neutral through the service enclosure is a grounding electrode conductor.