What is the best bed position? Under a window, on the same wall as the room door, or a wall with plumbing behind it?

1. In general, there are two methods:-

1.1. Based on Traditional Compass School of Feng Shui:-

You can browse this link for the various free reports:-http://www.geomancy.net/reports/reports.htm

The specific report that you should be looking at based on the compass direction is:
http://www.geomancy.net/reports/reports-list/perlist-sleep-position.htm

2. The second cateory which is either using intuitive means to locate the best bed position or commonsense Feng Shui :-

Itarina L. wrote:
What is the best bed position? Under a window, on the same wall as the room door, or a wall with plumbing behind it?

2.1. For the above, usually Para 1 is considered first. And see if we can try to place the bed-head in our advantage position. Sometimes it can be done; on other occasions: it should be considered as "NICE-to" have and not a "MUST-have".