I can't think of any one synth - hardware or software - that I gravitate to. I'll use "whatever" when initially working out the instrument arrangements to a song, and when that's done I'll go searching for the best instrument for a particular track. Could be software or hardware. Sometimes the original tracks might even remain if I got lucky on the initial tracks. 
 
The only common denominators I can think of is EZDrummer....It's fast, easy and lightweight to get things started, but it may or may be there in the end. For organ parts. I'll start with VB3, (which is fine "sometimes") but in the end if my Hammond / Vent / amp / sounds better, then VB3 goes bye-bye. Same for bass. Bass could end up being a Kontakt bass or the real thing in the end, but starting out I'll use a VST of some sort to work out the bass lines.