Not sure exactly what you are after. For electronic stuff a \'sampler\' wouldn\'t be my first choice, and the RMIV is really mainly about drums isn\'t it?
Monster softsynths with tons of features and effects are z3ta+ and Rhino. Albino 2 and impOSCar are also well worth a look, and far easier (at least for me) to program. They are very reasonably priced, and support from all the developers of these products is excellent (z3ta+ has just had a major (free) update, and Rhino will soon be going to version 2.0 (again free)). Copy protection on all these products is painless too.
Absynth 2.0 is also worth a look. It has a dictinctive sound. Not got a very extensive effects section though, but that doesn\'t stop it producing some very interesting stuff. Not as good value for money as z3ta+ or Rhino IMHO.
Spectrasonics Atmosphere could be described as a sampler. It\'s very easy to use and comes with excellent sounds (1,000 ish) that can be tweaked. You won\'t be able to design sounds from scratch like the other synthsizers though if that\'s what interests you, and you won\'t be able to load new sounds into it.
If you want to read more about any of them, they are all listed with reviews here, with an extensive forum where the developers can also be found.......
No point asking anyone which is \'best\'. Just like sample libraries, you\'ll get as many different answers as people you ask! Depends mainly on how you like the sound, and how you like the interface. All the abovementioned are excellent, and synth lovers could no doubt add another 20 products to this list without even trying. Fortunately, there are demos for most of these, so you can see for yourself.