Ryan Air is the cheapest
http://www.ryanair.com/
It's a good airline in the fact that it's not a fly-by-night cheap airline where you wonder if the plane will fall apart on take-off.
But it's cheap because it's no frills. It's just a bus in the air. You won't get drinks, and munchies and tucked up nicely by the flight attendent, etc. But you will get from a to b.