What's everyone's favorite ensemble? I think my favorite two at the minute are:
Resochord. You feed it audio and MIDI and the audio will resonate 6 virtual strings, the pitch of which is determined by the MIDI (you can also define chords within the ensemble but I find this less versatile). You can shape the sound of the resonators (envelope following, color, feedback, filters etc) and it just sounds AWESOME when you apply it to drums or percussion. I've seen other versions of this kind of thing but to my ears the one in Reaktor is the best. I've gotten some truly lovely sounds with this, including a beautifully out of control sustained guitar type sound. I've fed it drums, vocals, even low frequency noise, and gotten some incredible results. Hard to control sometimes but once you wrap your head around it it's a superb instrument.
Eccentrix. You can find this in the Reaktor user library. It's a really unique sounding FM synth and you can get some awesome sounds out of it. Sounds very close to a hardware synth.
Also worth checking out from the user library is a violin simulator called Serenade. It has a virtual bow that you can assign to a controller and it sounds very real, a spectacular piece of Reaktor programming.