Denorex with 2.5% coal tar works best for me (Head and Shoulders didn't help at all). I think this is a very individual thing, however, and what works for one will not necessarily work for another.
I've used a lot of them. Selsun Blue seems to work well for me when dandruff is particularly bad in the winter. Otherwise I can get by with any standard Pyrithione Zinc-based shampoo.
Head and shoulders has prevented dandruff for me for the past 10 years. I only use it once a week now as a preventative deal. Mostly using shampoo bars and still no issues