Tacko and Poirier have yet to play an NBA minute. RWIII doesn't look ready at all, and Kanter is a horrible defender. None of them have convincing arguments to start. Theis knows the system, and can move his feet. That alone make him a no-brainer, at least at this point.
Kanter has averaged 18.3 pts, 11.9 rebounds in seven games against Embiid (including a 31/22 effort), while Embiid averages 20.9 pts and 9.4 rebounds (shooting 47.7 percent).
I'm fine with Theis out there most of the time, but against a real big guy like Embiid, I'd rather see Kanter or maybe Poirier.