+1, one of the classic VSTis. Y' just sayin' or do you want something that is currently available?
GSI's VB3 is arguably the best emulation available, particularly if you are looking for a gritty sound. Its of the same vintage as B4II, IMHO they are pretty much on par except VB3's distortion is better. Main down side is it has not been updated in ages. I did hear rumours of a 64 bit version but that was a couple of years ago. Abandonware??? From the same developer the free Organized Trio is not half bad.
Very specific but nice and sleazy is
Sampletekk's Tonewheel It has just the sound I have been looking for. Some discussion
here Main down side - needs the full version of Kontakt.
NI's
Vintage Organs are very good with the exception of slight tuning issues on either the B3 or C3 samples (I can't remember which one), most people cannot hear it with the Leslie running. There is also an M3 and Vox and Continental Transistor organs. The down side is it does not do dirty as well as VB3. For real grit I have experimented with using the DI sounds and running them through an amp sim and LX122. There were some issues with drawbar automation at one time but these have been addressed.
I don't know much about UVI's
Retro Organs. It does not have variable drawbars which makes it no-no in my book even before the iLok is mentioned.
Linplug
Organ 3 is a decent VST but when i tried the demo I got the impression it was a Hammond like VSTi, rather than a true emulation. Its probably more versatile than VB3 but not as authentic.
Precision Sound has
Gospel Drawbars. another neiche product for the full Kontakt. I have not heard this one but in the demo video that are using it with XLS Labs LX122 which would indicate its weak in that that department. (Indeed the weakness with all the Kontakt organs is Kontakt's Leslie simulation). LX122 is great for clean but it does not get gritty enough.