If you're giving me an infinite range of piping configurations and an infinite range of slug speeds, densities, and lengths, then I'll say that any of the above might be considered appropriate, depending on your situation.

F2 = DLF * F1.

F2 = dynamic analysis force that results in stress S.
F1 = static analysis force that results in stress S.
DLF = Dynamic Load Factor

Any kind of multiplier for F2 performed in a dynamic analysis is a safety factor, not a DLF.

If you're asking for a DLF for any and all configurations, I will agree that 2.0 gets thrown around quite a bit. But I doubt anyone has compiled a book of DLFs for different piping configurations. It'd be useful to us.