Well, I go for Skyscanner. It has some nice features under the hood, if you poke around. But I use it in conjunction with Kayak as they both leave out a few. I always book directly from the airline.
And yes the point about signing up to newsletters will get you deals. Handy for some not for others.
The ultimate flight search should take into account that not everyone knows the name of every city in every country that has an airport. At least with skyscanner you can just ask it to find you the cheapest flight to a country