Texture

Okay, you're a stupid twat if you don't have this monosynth in your arsenal and you deserve to die. (If you weren't aware of the raves Texture has received, or even Texture's existence, then forget the previous sentence.) Simply put, this is one of the most unique and usable monosynths around, free or not. It's attactive and surprisingly professional-looking. It's got a ton of presets which are all wonderful. Has a wide frequency range. Only downside is that sometimes the parameters of a tweaked preset in a song will change when you re-open the project. So save those patches if you tweak them!