The one you like the sound of best!
If that’s an A4, great - but I have a very love hate relationship with mine. It doesn’t sound warm, deep, classy, silky or glassy. That’s not to say it doesn’t have a character, it does, but that character is nasal, wheezy, pristine and quite digital sounding in a way. It’s a pain to use (small screen and lots of menu diving) and it can be a real hassle to remember the various arcane butting combos and file structure. Sure you can learn all those things but then time elapses, you use other gear, or life / mood simply pulls you away from music for a while. When you return there’s always that uphill struggle of remembering how that fussy shit works. (The thing that stops me selling is the the CV, well played Elektron!)
I have a few nice synths, but funnily enough, the one that keeps the space warm next to my Digitakt is the humble microbrute. It’s cheep, compact simple and it just gives and gives. It has a very broad timbral capabilities, it can be aggressive (easily) or reigned in (a little more work involved) or you can just tame it inside the sampler.
I’m not suggesting a microbrute - instead I’m implying that any synth that you find to be aurally pleasing with broad timbral capabilities is the best partner for any sampler. Heck midi wouldn’t even be a concern for me - just sample some sounds and deal with them in the box.
That said, sequencing an external synth over the midi channels is versatile, frees up sample tracks and allows you to bring the elektron sequencer paradigm to something that would otherwise require a DAW to reach for the kind of complexity that Elektron’s sequencer can facilitate. (so maybe midi and polyphony could be a requirement?)
So far the Digitakt has lead me to a sonic space that might be described as broken down, consumer, 80’s hi-fi. That’s not a neg, and nor is it an objective statement about the sonic character of the digitakt (although I hear that character in a lot of other people’s stuff - crispy, toppy, a little pristine but artificially inseminated with wonkiness). It is what it is - it’s the sonic space I’ve been drawn to inhabit. And weirdly, that vibe has lead me to consider Dave Smith’s modern synths, despite that fact that I’ve been resistant to their sound.
I have a feeling that something like the new prophet series or sequential OB-6 would sound super dope with the stuff I’ve made on Digitakt so far - although It probably helps that I’ve slowly ( glacially) been warming to the modern Smith sound for a while now. (Oh - and I’m not saying Dave’s stuff sounds like wonky 80’s hi-fi - just that I can hear how the DSI character could really compliment my Digitakt output thus far)
A crucial point just occurred to me. Well in truth it actually occurred to me the other day when I watched a snippet of a video about a techno nerd collecting found sounds. Nothing wrong with found sounds - it can be a fun and rewarding experience just recording them - but as I’ve been focusing almost exclusively on the Digitakt lately, I was stuck with one thought in response… Why bother? I’m really not very picky about sound / sample source on the digitakt. It’s so versatile in its ability to warp a sound out of all recognition, that quite often I find myself with a “lame” sound only to have warped it into something awesome sounding a few minutes or even seconds later. On the whole that’s more true of percussive elements (well it is a drum sampler) but I’ve been surprised a few times when I’ve recorded a synth into it, only to completely destroy the sound with overdrive, bit crushing and heavy filtering (plus plocks) completely annihilating the original character of the sound but nonetheless it sounds great and sits in the mix in a dope way. Not being able to route channels individually into my desk and apply EQ seemed like a use case for the OT or rytm II, but honestly I’ve been able to shape things up pretty nicely in the box and I appreciate its simplicity relative to the OT.
What I’m saying is - the Digitakt is so versatile that I genuinely believe that you could just use the factory samples and still produce a lifetime’s worth of dope sounding tracks. Maybe then the best synth pairing is simply the one you already own