For a treatment shampoo I like Polytar, although they have changed the formula (surprise, surprise) and I don't like it as much as I used to. It does smell better than it used to though! I probably use this once a month at the moment.
For my regular shampoo the main thing that's important to me is that it doesn't contain SLS. I find SLS makes my scalp so itchy and flaky. Currently I'm using the coconut water shampoo by OGX.