MYTHOS! It is gorgeous (I normally don't like beer) I drank vast amounts of it in Kalami this year. Its quite good value aswell for a bottled beer because in the tavernas you get a big pint bottle for around 2.80 - 3.00 euros I think. The small bottles are a little more expensive in Cocktails & Dreams at 2.50 Euros (unfortunately they only do small bottles).
The supermarkets sell the big pint bottles for around 1 euro I think so its a bargain to have one or two before you head out.
Amstel wasn't to Andy's or my liking but lots of people were drinking pints of it, and it was the cheapest option at under 2 euro a pint.
Alpha isn't too bad either, but Mythos is definately our fave!