In some circles, there is much debate over what is best for a bench top. I see it as: it really depends on what you are going to be using the bench for.
If you are a hand tool user, you may want to go with a softer material for the top. reasons are, the softer top will help grip the work piece when clamped to the surface when a hold fast is used. also when the work piece is dropped on the bench, the bench will take the dings and not the work piece (in theory). Also it is easier to reflatten with a hand plane if it is a softer material. these are just some of the theories behind the softer side.
the hard sides have advantages too. looks, durability. bragging rights. the hardboard tops are good if you plan on having to replace the top in the future,
plywood, MDF, solid core doors are all suitable tops but keep in mind that these tops are only temporary. once they get abused enough they will have to be replaced / repaired. but they are drastically cheaper then a true solid wood top.
Its a catch 22. damned if you do damned if ya don't, what ever your personal preference is and your budget can afford.
personally, I think the best bench top is the one that is used and has stories to tell. and the scars to prove it!