It really depends on what you intend to wear them with. I prefer chestnut because the lighter sand color gets dirty too easily. I also like the contrasting lighter shearling inside the chestnut color (sand matches the fur and the black is dyed black). Black is versatile, but they will stain your feet and/or socks, which can be a PITA.
I love the sand color personally, but they do get dirty very easily. I have black as well, but they do stain your feet which sucks. I LOVE the chocolate brown color but my pair ripped and I had to take them back (and by then they were on backorder for 6 months). I just can't win!! My favorite is sand though, even with the staining.
^^ I agree.. I have a pair of boots that look a lot like Uggs that I got probably 5-6 years ago that are the same chestnut that Uggs does. I think the sand and black are very Ugg-ish and the most unique. The chocolate is great too.