Flux in general is probably the most important right after making sure you have an iron capable of handling the thermal recovery (you can't get the shit to stay liquid long enough if your iron can't handle keeping temp). The difference between relying on the flux core in even my leaded solder and adding a tap of my MG Chemicals flux when making a joint is night and day.
My lead-free joints come out nicely rounded and shiny just like my leaded ones with some practice and the right consumables. But I certainly have to break out hot air station more than I would when using leaded solder to deal with larger thermal masses (and why I will still break out my spool of leaded wire on rare occasion when I'm working with temp-sensitive components and just want it on the freaking board).
My lead-free joints come out nicely rounded and shiny just like my leaded ones with some practice and the right consumables. But I certainly have to break out hot air station more than I would when using leaded solder to deal with larger thermal masses (and why I will still break out my spool of leaded wire on rare occasion when I'm working with temp-sensitive components and just want it on the freaking board).