My personal, hands down, no contest, favorite iteration of the Ultraboost. The 21 has everything in balance for training and racing. No exaggeration, I have at least a dozen different 21s in my rotation. I stockpiled too many- even for me, so I’m releasing a few pairs. A beyond fantastic neutral gait shoe with ample cushioning without becoming pillowy. The neon green-yellow (or is it yellow-green?) accent color is so damn sharp. It makes you look fast even if you’re standing still!