Here’s a nice list of FM synthesis tools for Reaktor from the User Library.
WaveDuo is a monophonic/paraphonic wavetable synthesizer by Jonathan Tremblay. Available to buy and in the NI Reaktor User Library.
The Mountain is a free Synthwave subtractive synthesizer for 80s inspired sounds. Needs the full version of Reaktor.
Native Instruments offer a ton of sounds (2000) and instruments (Synths, Smaples Instruments) plus an organizer app (Komplete Kontrol) for free. Download here.