The problem I've found with big air coolers (like NH-D15) is thatthey don't really fit windowed Nano S. At firts, I was planning to go with be quiet Dark Rock 3, but despite being 160mm in height, it doesn't fit the case bacause of it's fan.
What would be a good water option? I like those:
Corsair H60 (enough?)
Corsair H80i
Corsair H90
NZXT x31 (how good temps would it deliever?)
NZXT x41