I know a guy who can turn a freeware filter/LFO and howling cat sample into dubstep.
Kidding aside (or was I?), you didnāt specify what kind of ādubstepā youāre interested in, so Iāll assume American-style, Beatport-oriented a.k.a. ābrostepā and not original UK dubstep / future garage / 2-step or post-dubstep.
So, āaggressive soundsā is what Iāll assume.
A good low-pass/hi-pass filter plugin (including Cubaseās) + an LFO will likely be your most used tool. That, and good sidechain / group bussing techniques (with EQ and compression, etc. on the busses).
Other favs from artists like Noisia, include:
Fxpansion Maul
Isotope Trash
Fabfilter Saturn and/or Volcano
My favorite Filter / LFO is Cytomicās The Drop, but itās very CPU hungry (but the circuit-modeled sound is amazing).
Itās less about the sound source and more about how you stack and group your sounds (and how you process that buss). That said, my favorite sound source is Fxpansion Synth Squad, but Cubaseās stock instruments work fine. Also, the Sylenth1 continues to amazing me with how warm and crisp it sounds. It really is an excellent synth (clichĆ© as that sounds).
In short, my recommendation is to put your money on good insert fx plugins first, before any synths.
Music radarās free plugin roundups:
http://www.musicradar.com/us/tuition/tech/the-27-best-free-vst-plug-ins-in-the-world-today-277953/1
The cheapest freeware synth can sound amazing if processed and stacked well ā bonus, you avoid the āpreset pitfall.ā
Cheers.