There’s many software solutions for editing soundbanks and building virtual instruments.
My choice felt on Reaktor for many reasons :
–  Reaktor can be used in stand alone or as a plugin ( VST/AU ), that lets a good compromise on portability, and it can be open in all kind of DAW (Ableton, Logic, Cubase, etc…)
– The sound engine of Native Instruments gives a reknown high-fidelity sound rendering. All the built-in modules (samplers, filters, oscillators..) can rely on a first class DSP.
– The interoperatibility for external communications is one of a most achieved. MIDI and OSC are entirely implemented.
– Reaktor is a powerful tools for a junior developers. Reverse engineering orientated, it lets the possibilities to maintain a simplified customization. Indeed, it can be easily updated. Final users can expand ensembles according to their needs with all the features they want.
– Even if “Blocks” are trendy since the release of version 6, Reaktor is still adapted for creating Grooveboxes, Synthesizers, Samplers and Sound Generators.

Last reason I choose Reaktor is because I used it for years. I’m still curious and attracted by other softwares like MAX/MSP, Max For Live, UVI Falcon, NI Kontakt, Pure Data..
May I will experiment and release projects with one of them in a close futur.
But Reaktor still the one I am the most able to master for designing sounds & GUI.
Last but not least, I really feel myself close to the community that revolves around it.

If it’s not already the case, you can get your copy on the Native Intruments website.