I am currently using nTrack in conjunction with my keyboards, but I don’t want to have to take them with me wherever I go.
I downloaded some free VSTi from the web so that I could learn how they work within nTrack, but now I need some that are sonically better.
What I need are some good pianos, organs, bass guitars and some good pad synths.
I’ve had a look at a the IK Multimedia sets (SampleTank and Expansion Tanks) and USB Plugsounds.
Can anyone point me in the direction of any other good plugs? 
You looked through the reviews on kvr-vst.com?
The Native Instruments stuff is killer.
For analogue sounds, I really like the Pentagon - and it comes with a stand alone ASIO microhost. Those folks have some others that are really amazing. On the original Triangle (free monophonic) the filters sound pretty darn close to real to me.
Also standalone are the synths from Big Tick - some of those are free - I dig the cheese machine, although you might not. I also think Rhino is pretty amazing.
It depends on how much you want to spend. For free, there is a ton that can be done with free SoundFonts and a VSTi SoundFont plugin like sfz. Check out Natural Studio.
If you don’t mind spending some cash SampleTank is a pretty good all around plug in package. Synth wise it may leave you wanting though if you are big into making your own sounds. You might be better off with one of the zillion or two FM/subtractive synth plugs out there. There are so many of them it is hard to recommend one. But I will say the NI stuff is very nice form what I have seen. I’m not much of a synth guy myself, so I am not the right person to be suggesting one over the other. Edirol also makes a few general purpose piano/guitar/bass synths. Steinberg has a free bass VSTi called VB1 and they make a few synth and drum packages and a nice piano.
On the top end, there is also the big boy sampler packages out there like Gigastudio, Kontakt (from NI), Halion (from Steinberg), etc. With these you can use massive sample libraries from a lot of developers and they sound great. However, you can spend a lot of money very quickly.