Purple Mist is a personal favorite of mine since it focuses on the ambient capabilities of Fathom Synth. This is a great bank for both beginners and advanced sound designers since loading each preset provides a live tutorial on how to get the maximum ethereal qualities out of Fathom.
The reason the Scrubbing Monkeys banks are so popular is because of the amount of detail he puts into each preset. Each one has been carefully crafted to be the kind of sound you can build a hit around, especially if you are creating any kind of electronic dance music or ambient tracks. Moreover the designer has a cool habit of programming the mod wheel to morph the sound into an entirely new set of audio characteristics.
The Scrubbing Monkeys banks also tend to be very popular with film score artists due to the sonic detailing. If you use Fathom Synth regularly the Scrubbing Monkeys banks are an absolute must.