I am curious on what the simplest possible rocket design that have been proposed which could reach orbit. My first thoughts would be earlier rockets like the Atlas LV-3B, but then I consider the stranger designs by OTRAG, through those don't appear to be confirm to be able to work.
Depending on exactly what you mean by simple, the SS-520 might count (see the later SS-520-5 for a successful launch). No guidance, just point it and let it go and stage for a bit. As far as I know it's still the smallest orbital rocket. Not that small implies simple, but the SS-520 happens to be both.
EDIT: looks like I was incorrect; the second stage also does a one-time re-direction using cold gas thrusters, which requires a full GNC suite. This rocket isn't that simple at all. I'm a liar! Take those upvotes away