For a basic filter, the Doepfer Wasp is great filter than can self oscillate and get wild or be tamed down. Filters are loads of fun.
-- sacguy71
For completeness, I need to mention that this is actually not true of the Wasp filter as it is sold by Doepfer, self-oscillation is possible but not without modding the module or using other tricks past simply turning up the resonance...It's actually the only one in my rack that doesn't self-oscillate, but it sure can get wild and filters are sure to be fun as well :-)
--- Voltage control all the things ---