I would choose the Brother 888. It seems to be threaded from the side, and such machines in general do a better stitch and you can sew heavier thread and fabric with it. This machine got a lever to position the needle left or right. The Brother 651 do not have that. I do not know about the last Brother.
But the Brother 888 is a flatbed machine. A free arm machine can be handy for repair work on clothes, so the Singer 431G can do that, but it do not have the other advantages, that the Brother have.
If you can find a Singer 237, it would be a good and reliable machine as well and often you can get them to a lower price.