Which is more accurate for diagnosing prediabetes fasting glucose or a1c?

Both have their role. The A1C reflects an average blood glucose over a 3 month period while a single blood glucose is a today/now level. A single high blood glucose , unless it is very high ,may not give enough data to be diagnostic, but can require further testing to be sure. A high A1C would represent a more valid indication of pre-diabetes or even active disease.

Fasting plasma glucose testing done at an accredited laboratory is more accurate. A1c is affected by other factors, in addition to plasma glucose levels.