I was shooting a kayak race with a Nikon D3 & Nikkor 200-400mm f/4G ED-IF AF-S VR. Between runs I noticed a pretty girl in the gallery nearby and without giving it much thought I turned and snapped a candid shot. While editing the shots later I was blown away by the fine detail captured in the photo. It's not a controlled test or scientific lens comparison, just a "wow" moment.
I'd have never thought of a 200-400 zoom as a portrait lens but it did a pretty fair job on this grab shot. :)