Using SAC305 mostly. However, this is only half of the story. Secret is in the flux. LF4300 is a good REL0 flux for example. Amtech also makes SAC 305 wire with it. However – even as it says no clean on the label, you do want to clean it if it ever goes to work in humid environment (learned it the hardway – product series recall).
When it comes to wire and good looking solder joints, you want something which splatters less (read: has less flux in it). Yet this is a compromise between splattering and wetting…
Non-water soluble no-clean fluxes are quite good. Chipquick SMD291 and AIM NC for example.
A good intro to the world of flux[es]: https://www.pillarhouse.co.uk/technology/choosing-flux-technical-article