As I am an upright bassist, first and foremost, the top two would be upright acoustics:
1) A "4\4" (actually 7\8's) Bjorn Stoll "Dresden". Flat back, Poplar ribs & back table:
2) A reproduction of a flat back 5-string Kaakstein or Ferdinand Seitz. Stefan Krattenmacher's Kaagstein replica shown below:
The other four would be bass guitars, of course.
1) My G&L LB-100, strung with stainless flats:
2) A Geoff Gould GGi4 with Jazz bass pickups, quilt maple top, with a custom "Joker-burst" (lime green fading to deep purple edges) finish. Not illustrated in photo below, but still looks nice!
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3) MTD 535-24. Not sure what woods I'd pick or finish I'd request. But the one below looks pretty nice!
4) Dingwall Z3 6-string. Again, not sure what options I'd order, but it would be a solid finish with "Tachometer" inlays, of some sort.