Pretty much everything by Sigmar Kreie
NATIVE INSTRUMENTS : Community : Reaktor User Library
Especially weedbacker, FFmer, lookuplab, clusterbasher.
Also WeedwackerR5 v1.0 that is an update to Kreie's weedwacker/backer that is one of the most beautiful/bizarre sounding synths there is.
If you build, the filter in Stephan V Viral Extinction is a must because he shows how to do some crazy stuff with control shapers on rez.
I also think its worth downloading everything by Dieter Zobel in the library...sometimes very hit or miss but always quite weird and different.
Acoustring is also a must, such a beautiful physically modelled string/weirdo guitar.
miGraine / Marlene v2.0 by gabriel mulzer is also a very strange and beautiful kind of string synth but built only using formant filters.
For emulations, there should be a folder on the R5 disk from 2.3 of minimoog, sem, juno(original version of junatik)sh101..The emulations sounded better with 3.0 though because the oscilators were much more "raw" than now.
Spark is good, but to me if its worth buying it at all its for its distortion which is just fantastic and can be easily put into other synths.