The first Doepfer filter that i ever bought was the A-121 Multimode Filter. It is a 12 dB/Octave filter, and has multiple simultaneous outputs available; a Low-Pass, Band-Pass, High-Pass and Notch (or Band Reject).
It also has 4 CV inputs, 2 for adjusting the cutoff frequency and 2 for resonance.
This is still one of my favorite filter modules, the multiple outputs are very useful, and i like all the CV inputs. It just gives you so much more control over the filter, and that's what i like most in a module; maximum control.
Although it is 'only' a 12 dB filter it sounds very nice, from very smooth and warm up to quite rough, especially with the resonance turned up ( ...up to self-oscillation ).
Most of the times i patch an A-174-2 Wheels CV into FCV1, and a simple envelope CV to the (adjustable) FCV2. Works fine in most situations.
With some analog or digital noise added to QCV2 to give it a touch of unpredictable resonance you will quickly get a convincing fat bassline.
Together with an A-138 B (exp) Mixer you can easily make some pretty impressive stacked bass-sounds, all with just a single filter-module.
Find A-121 sound examples at Andreas Krebs Blog : HERE
As the special circuit CEM3320 used in this module is no longer available the module is discontinued. I believe there are still a few available, but they don't make new ones anymore. Doepfer advices modules like the A-106-6 ( with eight simultaneous outputs! ) or A-107 Multi-Type Morphing Filter as a replacement.