With All-on-4 dental implants, what's the best and most natural-looking material for the teeth: ceramic, porcelain, or zirconia?

The two most common materials used in All on 4 style cases are acrylic and zirconia. Although zirconia tends to give the most pleasing aesthetic result, it is a very hard material. Consideration must be given when both arches are being restored. It has been reported that when zirconia teeth on the top are opposing zirconia teeth on the bottom, it can actually produce a clicking sound when speaking. Another concern is that such none-yielding materials can transfer stress to either the abutment connection or the implant itself. Proper diagnosis and treatment planning should be done on a case-by-case basis. It wouldn't be a bad idea to consider zirconia teeth on the top for aesthetics and acrylic teeth in the bottom to cushion the force. 

Hello 1joel2u, 
The most life-like material that we have in dentistry today is porcelain. Remember that porcelain and ceramic are the same for the purposes of our conversation here. A dentist will use the terms interchangeably. 
In regards to All-on-4 dental implant treatments, the choices are - Acrylic Teeth, Porcelain Teeth, or Zirconia Teeth or a combination of the above. The strongest of all with the least amount of complications is zirconia. It also comes VERY VERY close to the aesthetics of a porcelain crown that the naked eye cannot discern the difference. 
It is also important to keep in mind that one material doesn't solve all problems. Some patients are not zirconia candidates so I highly recommend you see a prosthodontist (prosthetic specialist) for an evaluation and proper planning.
I hope that answers your question.