Soulsby Synthesizers Atmegatron by dtronik
In the interests of fairness I'll declare something upfront; I love the Atmegatron. This little mono synth has a vibe all of it's own; it may not be hi-fi (it's 8-bit) but it's capable of a surprising range of sounds - often crunchy, sometimes filthy, occasionally eerie or even weirdly beautiful.
That uniqueness is characteristic of Paul Soulsby's design - the unit itself has a quirky appearance and the layout is straightforward but effective. And it is playable to a ridiculous level; SO immediately tweakable with just the right balance for sound design (which to me is right in the middle of science and wonder - think "mad professor's lab" meets "voyage of discovery") it'll keep you coming back to it regardless of how many synths you already have.
But that isn't even the best bit. What makes the Atmegatron stand out is the concept of being hackable; now I am not an Arduino programmer (although might have to learn to be!) and coding your own synth may not be of interest - but Soulsby Synthesizers already have a range of source code downloads available including the Atmegadrum (8-bit drum synthesizer), Atcyclotron (PPG-esque wavetable cycling) and Delayertron (sci-fi delay fx) available free which you can upload any time you like. So it's not just one synthesizer; it's any number of synthesizers, and a great user interface to be able to use it. Seriously, for less than £200, what's not to love??