Hi Diki
I currently use various versions of the following:
True Pianos
SGM180 (Old Soundfonts but with some nice sounds that go well in a mix)
Battery 3 (Trial as I am still undecided whether this, or to go for EZDrummer)
Independence Pro
I have also various smaller ones from the KVR site.
Currently looking into Kore 2, (As all other VSTi can be incorporated within it) but still not sure whether to have the Hardware Controller as well. (Plus the new deal NI is offering with Komplete 6 is also tempting, although interestingly the Akoustic Piano is missing)
I will also be moving on to a Multi-Core CPU and 4GB of Ram.

Before making any firm decisions on the above, we are overdue for Drums 2009, which if rumours are correct, will actually be a complete new drum module for the Wersi, and will replace the current 7 yr old one which is way past its sell buy date.
Regards

Bill