Here is the official seating plan for KLM MD11s. Slight differences between aircraft are possible (according to the disclaimer), but on their 737s I've never seen any.
Sometimes, the last few rows of businessclass seats are seated as economy depending on the number of businessclass tickets sold. Those would be your first choice (but check in very early to get them, as every regular flyer knows this).
Second seat from the left or the outer seats in the middle section seems the best to me. Aisle to stretch your legs and only one person to your side.
Aftmost portion of a trijet is usually quite loud because of the engine being near, and you might want to get a row near one of the galleys so you get served more quickly