I’d have to say the Octatrack.
Not only is it my favorite piece of hardware, but you dont have a sampler in your list.
That said, the OT is not just a sampler, it’s also a midi sequencer and effects box.
I would say the OT is also the most versatile of the Elektron machines, probably the most complex to learn as well.
So if you get the OT you’ll have something you complimentary to what you have, and learning it will show you what the elektron thing is about.
The OT was my first elektron box.
Shortly followed by the RYTM and A4.