I won't try and say which is best', but I will say which 'are best'. and there are probably a couple missing.
Stock out of box the Thermalright Silver Arrow SB-E Extreme (TY-143 fans) and Silverstone HE01(FHP-141) are best by a couple of degrees, but only because they come with super high cfm super noisy fans. When you slow the fans down to normal speeds they perform with the rest.
Cooler size comparison (Width x Depth x Height looking from front of case with motherboard laying flat)
Akasa Medusa .= . 8x 6mm pipes 144x129.5x163mm w/ 140mm & 120mm PWM fans 49.6mm*
K2 / Assassin . = . 8x 6mm pipes 140x128x158mm 146x154x160mm w/ 120mm & 140mm PWM fans43.2mm*
Dark Rock Pro 2 =. 7x 6mm pipes 133x124x166mm 133x149x166mm w/ 120mm & 135mm PWM fans
NH-D14 . . . . = . . 6x 6mm pipes 140x130x160mm 140x158x160mm w/ 120mm & 140mm fans44mm*
NH-U14S . . . = . . 6x 6mm pipes 150x52x165mm plus fans base offset 2mm 41mm*
PH-TC14PE. . = . . 5x 8mm pipes 140x159x171mm w/ 2x 140mm fans, PWM adaptor40-45mm*
Megahalem. . = . . 6x 6mm pipes 130x74x158.7mm plus fans 40mm*
HE01 . . . . .= . . . 6x 6mm pipes 140x119x160mm w/ 140x38mm PWM fan 42mm*
Archon SB-E. . = . 8x 6mm pipes 155x53.6x170.2mm plus fans 34.45mm*
Silver Arrow . = . . 4x 8mm pipes 147x123x163mm 151x149x168mm w/ 2x 140mm Fans 40.87mm*
SA SB-E . . . .= . . 8x 6mm pipes 155x105x164mm 170x130x170mm w/ 140mm & 150mm PWM fans 31.9/39/46mm**
SA SB-E Extreme . 8x 6mm pipes 155x105x164mm 155x130x165mm w/fans Smaller because of 140mm fan vs 150mm fan.
*cooler base to bottom fin. (add 8mm for motherboard to bottom fin.)
**fins are at 3 different levels
K2, D14 & 14PE are about same size cooler. 14PE has 140mm front fan
140mm fans can be problematic with ram clearance.
All can interfere with PCI-e slots 1 & 2
SA SB-E is widest by 15mm. 170mm with TY-150 fan in middle. TY-150 is 169x151mm
SA SB-E Extreme is 155mm wide with TY-143 fan in middle. TY-14x fans are 151x140mm.