In a quiet, closed space unhindered by distractions.

For me (and I'd imagine, many others here), constant conversations in the background, people walking by, etc absolutely kill my focus and keep me from getting work done. Working from home helps a bit, but my neighbors provide plenty of their own distractions.

Honestly, I'll take a broom closet with a laptop, headphones and an Ethernet jack over any other open office design.