To me this is two different questions – which produces the least feces, and which is most absorbed.
There are various real-life situations in which reducing fecal output is a necessity. In those situations, a diet that induces extreme constipation will, in the most literal sense, "produce the least feces", for a while, at least (potentially days up to a week or more). Banana Fried Rice, anyone?
The latter case, diets in which foods are most absorbed, are really much more likely to be "diets in which most of your food is water"… Things like watermelon, or celery (remove the strings first). Though there are some standouts in the "real foods" category – rare red meat, eggs, and goat's milk are all highly nutritive and add little to your "southbound mass". Raw, as opposed to cooked corn is also said to be highly digestible, though I refuse to believe it based on local outputs during the season. Certain starches, like potatoes (peeled), also have some potential to approach a high absorption ratio, though a lot of it depends on the means of preparation. Oh yes, and blending just about anything down to liquid form will help, you know, a LOT (though really you're just trading the door for the window, so to speak).
I'm hesitant to work up anything approaching a full diet plan, though careful study of the Atkins and South Beach diets should get you about 2/3s of the way there… Then just work to balance what they see as the "gainers" with those foods that are hard to absorb. And, you know, don't do it!
There's a great reason your body wants to push waste through you on a regular basis, and when gamed for long periods, colo-rectal health issues may arise. I'm not a doctor(*), but with the exception of avoiding embarrassment during an imminent anal porn shoot, there are very few – if any – good reasons to use fecal reduction as a goal in one's diet.
deano.com 