Traktor Pro

Traktor is hands down by far the best DJ software there is. It can be as simple or as complex as you want. Beginners can figure out how to use it fairly quickly, and professionals can customize it to their hearts content.

Traktor simply does some things that no other software does. Stems is the newest one, but there have been several other features that they beat everyone else to market with. Having said that, they also lag behind the rest of the industry with some features too. I've been using Traktor since public beta back in the 90s, and it has seen long stretches of inertia when nothing was updated and the development seemed to stagnate. Nowadays they are pretty good about putting work in. One thing worth noting: Native Instruments is really a hardware/VST company. Or at least that's their "bread and butter", so having high expectations of Traktor (especially using third party hardware) may lead to disappointment.

Best audio app ever. Cool tip: If you want to use a device with traktor that's not MIDI, you can download "Xpadder" and map the controls to your keyboard, then map those keys to traktor. I've even used a DDR Dance mat for extra controls.