Best cooler is a custom loop but if your more interested in plug and go then the best 2 out there are the open loop systems Swiftech H320 X2 Prestige and EK Predator 360.
Cooling is the sum of all parts... A great cooler will only do so good if you have crappy thermal paste and or bad case cooling. Noctua D15 is great and has arguably the best performing fans in the industry.
Liquid cooler with the largest radiator space is going to give you the most cooling performance. something like a fractal S36 is probably the way to go for you. Maybe an EK Fluid gaming kit.