From our VST instrument experience, EVERYTHING.
Nowadays we tend to do everything with VST instruments, Gets rid of a mass of "noise" from real-world synthesizers, modules, etc.
There is a useful list of freeware VST instruments at:
http://free-plugin-list.com/free-vst-pl ... show_cat=1
From General MIDI/GS sounds we use the Edirol (Roland) Virtual Sound Canvas, comes with 902 sounds (bass, pianos, organs, strings, etc) and 26 drums kits. This VST software is pretty cheap nowadays.
http://phoenixdesigns.org/music2006/hocuspocus.mp3 (All Sound Canvas sounds)
For more electronic timbres we use Arturia's Moog Modular and ARP2600V. All those knobs and sliders can be controlled by MTS as well. On a good day you can fool yourself into thinking you are Wendy Carlos
http://phoenixdesigns.org/music2006/bach2006b397a.mp3
For more modern sounds we go for Linplug's ALBINO 2 and 3, and Pro-Sounds PS-1 Performance Synthesizer. Great arpeggiating effects, gutteral basses and sweet timbres. Fully programmable to boot......
The EVM Synth Bundle has also proved useful. Not classed as high-end, but does allow for some interesting and unique sounds. Sweeping pads seem to be their forte......
Nick B