Zerg are my favorite race for 1 on 1s, heck, look at their stats!
Attack-8 Defense-4 Expanding-9 Adaptability-7
The Zerg are masters of rushing and easy to zip towards high-tech (if
you can call it technology) units like Guardians. Several strategies for
attacking are: Guardians and Devourers, mass Hydralisks, mass
Zerglings, or mass whatever really. MY personal favorite is the Guards
and Devs, plus fully upgraded Zerglings. Zerglings pretty much suck
unless fully upgraded, but do that and they rock for their measely 25
mins apiece!