I think that is going to depend on who you ask, the actual machine and what you are going to do with it.
There are people who think a Singer Featherweight or a Singer 301 is the bomb. If you are going to do a lot of zig zag work those are not the bomb. If you get a machine in bad shape it is not the bomb no matter what it is unless you are kookie like me and like to mess with the bad machines.
Are you going for looks or for smooth even stitching? As far as collecting - anything goes. There isn't a whole lot of value in old sewing machines so look around at what is on QB and other places and see what might remotely appeal to you and your tastes then dive in the deep end and enjoy yourself. You may find some work better for you than others.
I thought I didn't like long bobbin machines until I tried one. Muv has wonderful information and that was the missing piece for me.