What determines what type of short-circuit current is highest?
A power transformer will commonly have a lower zero-sequence impedance than positive sequence impedance. See the J&amp;P Transformer Book. Typically a transformer will have \$ 0.8 Z_1 \leq Z_0 \leq 1.0 Z_1 \$.

If the zero-sequence impedance is lower than the positive-sequence impedance, then the line-to-ground fault current will be more than the three-phase fault current.



Generally in my experience the line-to-ground fault current is no more than 1.2 times the three-phase fault current. Depending on the system earthing, the line to ground fault current may be much less than this.

Impedance earthing is commonly used to limit earth fault current and hence touch/step potentials. So it's not unusual to see a switchboard with 25,000 A three-phase prospective fault current, but only 500 A single-line to ground fault level. On certain low-voltage substations with fault levels of 10 kA, I have seen the earth fault level limited to as little as 4 A.