RE: How to get a perfectly flat (=plan, leveled) workbench?
Yes, there are obviously many things you can use. In fact, I have a total of 6 workbenches and they all have different tops on them.
A Butcher Block is outstanding, but expensive. Ditto for a solid core door. But if you can find either of these at a good price, go for it.
Anything that starts flat will stay flat with the right support, so counter tops, doors, desktops, etc are all good.