I have large Armies in 20mm, almost exclusively plastic. Hat, Italieri, Airfix, Zvezda and even some old Atlantic stuff. They are fleshed out with a few Newline metals. As one might expect, they vary somewhat in quality, particularly the old Airfix stuff, but they look OK en masse.
If forced to pick one, and only one, manufacturer, I'd go with HaT as they produce figures for wargames, not primarily as toys.