my favourite sound is the "evil" acid lines...program ur 303 or audiorealism using notes C, C# and G# for that creeping evil vibe...the best sound i find at the moment is running Audiorealism on a square wave setting through a mild chorus, a medium length reverb with plenty of pre delay from Guitar Rig, then through a tape delay in PSP84 and finally limiting it.

heres a clip i just whacked out to show you what i mean:

http://www.dave-elyzium.co.uk/media/acid.mp3