I know a guy who can turn a freeware filter/LFO and howling cat sample into dubstep.
Kidding aside (or was I?), you didn't specify what kind of "dubstep" you're interested in, so I'll assume American-style, Beatport-oriented a.k.a. "brostep" and not original UK dubstep / future garage / 2-step or post-dubstep.
So, "aggressive sounds" is what I'll assume.
A good low-pass/hi-pass filter plugin (including Cubase's) + 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:
Fabfilter Saturn and/or Volcano
My favorite Filter / LFO is Cytomic's The Drop, but it's very CPU hungry (but the circuit-modeled sound is amazing).
It's 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 Cubase's stock instruments work fine. Also, the Sylenth1 continues to amazing me with how warm and crisp it sounds. It really is an excellent synth (cliché as that sounds).
In short, my recommendation is to put your money on good insert fx plugins first, before any synths.
Music radar's free plugin roundups:http://www.musicradar.com/news/tech/fre ... t-255880/2http://www.musicradar.com/us/tuition/te ... y-277953/1
The cheapest freeware synth can sound amazing if processed and stacked well -- bonus, you avoid the "preset pitfall."
Main DAW: Cubase 7.5 64-bit, Widows 7 Pro x64, Ivy Bridge 3770K OC 4200 H100 Liquid Cooling, 32 Gigs DDR3, Radeon HD 6870, Two 30" Apple Cinema Displays, Focusrite Saffire Pro 40; Studio Accessories: ATH-M50 Headphones, Korg Radias, NI Kore 2 Controller, 2 NI Traktor F1 Controllers, iPad Lemur App; External Optical Mastering Chain via a Mac Pro running Reaper, MOTU 896HD, Two 20" ACDs;
Mobile: 2009 Macbook Pro Running Latest Cubase 64-bit in Windows 7 x64 Bootcamp