Depending on what types of music you want to make, here's a couple of options:
Vanguard- the ultimate trance VST. Pretty much the only thing it's good for, though.
QuadraSID- Great for C64-style chiptunes.
z3ta+- A great, all-around synth. Supposedly it's mostly used for trance, but I've heard it used in many other genres.
I know there's some that I've forgotten, but in FL's bundled demo synths, Sytrus is pretty good, despite using massive amounts of CPU.