Don’t’ expect a simple answer – that depends on your application.
If you are looking for a solder paste, NC-SMQ80 is the flux vehicle of choice. For spray applications 5RMA has a strong following on Au substrate applications. For niche ball attach or specialty processes needing a tacky flux, WS-363 delivers the best wetting on ENIG (see attached chart), but WS-575 has a more useable rheology for pin-transfer. PoP Flux 030B is your ticket in no-clean applications at these low temperatures.
Did I mention all these options are Halogen-Free?