There literally is no best one.
Tal and dblue's older versions are free.
DSK
Keyzone, Dude keyzone is dope.
Zebralette is one of the strongest free ones out there period.
not sure if tyrell is still freeware though.
then of course there is also waldorf wavetable creator too and csound.
But seriously though there is way more than that to list so follow his link, check out vst4free and kvr for all of that info.