With us who shave their heads I wouldn't think shampoo is overtly better than a good soap bar though can help in some instances.
For the majority as said, a gentle soap like Dove, Cetaphil or good anti bacterial soap should be fine.
For those who have an oily scalp or sport a buzz don't have to but can try a clarifying shampoo like
Suave Clarifying shampoo – cheap
Menu-U scalp wash - concentrated gel, last ages
These will regularly keep the gunk off your head.
For severe cases if you've got issues like the above or irritation, pimples then something like
Head & Shoulder with Hydra Zinc – Good for Pityrosporum Folliculitis
BaldGuyz Wash – not exactly sure what about this works but it works in helping the scalp keep clear.
a Tea tree shampoo like American Crew Tea tree shampoo – great for oily scalp, irritation like pimples and such
Trumpers Valerian Shampoo - Only mentioned it coz I got it part of a Christmas present but like the above ,works as a soothing wash against scalp irritation.
Like I said though this isn't really required for the majority of us and isn't definitive, there's similar stuff out there to these things.