What are the best noise reduction plugins? I suppose it doesn’t have to be open source, but that would be nice.
Noise reduction is normally best achieved in dedicated stand-alone applications. You should take a look at gnome wave cleaner (or gwc for short).
Another option, depending on the type of the noise and the wanted signal is simply gating and/or eq’ing the signal. Gate to remove noise from places where there is no signal and eq to attenuate the noise throughout the signal.
Audacity is your best option here. Free, open-source, cross-platform and high quality, reliable noise reduction with many options to ensure you get the sound you need after the noise reduction filter runs. See more and download at https://www.audacityteam.org/
In context of Ardour as plugin (also cross platform)