Nothing beats the keys (be it a piano or a keyboard) - I start with them all the time (but end up switching to different instruments), they are the most versatile musical thing, you can play just about anything on them, improvise easily and they're quick to learn
-For Drums its Steven Slate Drums 4
-For bass , check out the kontakt library of OrangeTreeSamples (cherry bass) -For Guitars...........well check out our guitar vsti at Amplesound.net (will make an annoncement about it soon) Make sure to try the unlimited 7 day trial of AGG ( Gibson Les Paul Custom Guitar )
The question not the theremin I suppose it depends on the performer and which performance? it's sort of on par with when someone asks you an impossible to answer question like what's your favourite song, it's just too damn hard to answer! maybe the voice, possibly guitar, could be percussion/ drums, definitely not a Jews harp!