want to get real nasty.. dont' fuck around.. just go with metasonix
i sold my original pro-co rat. it was a fucking beast but i found i enjoyed the sound of the Pro-co You Dirty Rat more.
i also use my Future Retro XS's filter since it has overdrive.. this distorts beautifully, and i use the microwave XT's waveshaping filter with a wavetable table that has excellent waves for distortion... i really like using filters for distortion since they can be modulated
i recently purchased Ohmicide and have been pleased so far because i've gotten distortion out of it that i haven't gotten from any of my other sources... it is highly configurable... this thing is more than just distortion.. it is equivalent to an instrument in my opinion. you should check this out if you don't mind using software because it has a slew of distortion options that you can use on particular frequency bands... ever try to distort a bass sound? it looses a lot of it's punch and character.. well with ohmicide i don't have to run the sound through a highpass filter and then distort that and then recombine the distorted highpass of the bass with the original bass sound... you can have like 4 different distortions all working on different frequency bands all with their own unique settings
there are lots of different types of hardware distortion too. bitcrushers which reduce the sample rate, waveshapers/wavefolders, tube distortion, clipping diode distortion
the Sherman Filterbank is a beast for distortion too and will probably be my next purchase... i need more space and another rack