I enjoy the later poems the most, where Merwin lets loose a bit concentrating less on form and punctuation and more on his interesting way of being political.
Some of my favorite poems include Yesterday, Ash, Finding A Teacher, and Strawberries.
As an undergrad poetry student, I've been told about strong line breaks, punctuation, and show vs. tell. Merwin often disregards these facets. Sometimes it works, sometimes it doesn't for me.
As can be told by my favorite poems, my favorite Merwin moments are when he uses that loose, sparse style with a cool image.