The "Open Source" requirement was removed last year.

--jeremy

Voted RPi4B, though presently still using RPi3B+ as the RPi4B still doesn't boot from USB.

The Raspberry Pi 3 Model B is better with software than others.

Raspberry Pi 4 Model B

Jeremy, I own 4 different SBCs and none of them are in your poll.

nextthing C.H.I.P. (now a dead company).
Firefly RK3399
friendlyarm/friendlyelec Nano PI Fire3
Radxa Rock PI S

I'd say that the Nano PI Fire3 is the most impressive (it's on sale as we speak also!)
EDIT: When I say impressive, I don't mean the HW. There's lots of boards out there with better HW but worse support and drivers. My statement was for the combination of the two, as in, it has the best SW support of it's great HW of the boards I've bought or considered owning.

I already have too many pis.
Additionally, I have several cards I can't remember.
Asus, cubie, beagle bone, ...