Nizoral 2% is defiantly the most effective shampoo for combating scalp irritation/dandruff because basically it's the only shampoo on the market that has 2% keto. Yes you don't need to use it as much as other shampoos - it's very powerful hence you only need to use it once a week. My advice is to follow the instructions on the bottle eg use it 2 or 3 times a week for the first couple of weeks and then once a week once the dandruff/irritation has cleared up.
I've also tried Regenpure DR and in some ways it's better (doesn't dry your hair as much, no sulphates, can use it daily) but purely in terms of combatting irritation you need the 2%.