I have three IRS's on board and an FMS that will take information from every available position source - DME/DME, VOR/DME etc and if I look at the sensor page the one it is actually using is the GPS. It seems that as long as the GPS is in agreement with the others the FMS is actually using the GPS position. So I suspect that the GPS is the most accurate. I use it for precision approaches so it bloody better be!