Depends where you are staying as there is Nissaki village and harbour, the main road area and Nissaki Beach Hotel. We always stay at the village and all the tavernas are good - favourite for us is Mitsos as they are so friendly and remember and welcome you year after year. Food is good, fairly standard taverna stuff. Setting stunning at the beach, jutting out to the sea.
Best for food and classy atmosphere is getting the boat transfer to Agni, a really memorable meal but more pricey than local. Other local tavernas are the Olive Press, Nissaki Sea View (better just for drinks I think), and the one overlooking the beach (?Eleni) can't remember the name - nice view. But really you can't go wrong in any of them, very personal choice. Other people will have different opinion to me.

We are staying top side of road that runs through Nissaki, thank you for our replys much appreciated

Went to Roumellis because he does Greek Dancing on a Saturday night and it was so good we booked for the following week! First Saturday we walked from Barbati to Nissaki (did I enjoy that cold pint of Mythos when I got there?) so Vangelis the owner got one of his waiters Gregory to give us a lift home, that is what I call service.