Personally, I'm a fan of Doepfer's A-106-1. It's their take on the Korg MS-20 Sallen-Key filter pair with some extra nastiness (fun on percussives) and an insert point in the resonance path, which opens up all sorts of abuse potential for mangly things by altering the resonance circuit's behavior, either with or without CV control on the altering device. Consider, for example, what sort of trouble you could cause with just a little mono delay line in that path...