I find that the larger sows are the absolute best. 400 to 800 lbs and around two to eight years of age. My absolute favorite cut of pork is the Boston Butt steaks and Country Style Ribs (both the same meat, differently cut) from big sows. By this age they have strong marbling and rich flavoring. Note that this is on pasture with whey as a supplemental feed. On other diets it may be different. The catch is the cuts are huge - more like a steer than a pig. Our sows have about 1" to 1.5" of back fat since they're on pasture which is a low calorie diet.
Our adult boars are also delicious but leaner, less marbled. They taste more like
beef than pork. Our boars have about 0" of back fat. They look more like Arnold Schwarzenegger than the fat boars I've seen in some photos. The difference is the pasture and low calorie diet.
The typical market pig is 250 lbs or so because that is where they start putting on marbling and the rate of feed to meat conversion drops. This also represents the typical size many pigs can get to in one warm season thus making them fit with fall or early winter slaughter. Our finisher pigs have about 0.75" (boars) to 1" (gilts) of back fat.
Note all back fat measurements include the skin. On a high calorie diet all of these go to much higher back fat as we've seen with people who buy weaner piglets from us and put them on either whole
milk (4" of back fat on gilts at 8 months) or commercial corn/soy based feed (1.5" of back fat at 6 months). What you feed determines the flavor of the pork. Flavor is stored in fat and accumulates in about the last month before slaughter. Inter-muscular fat, marbling, makes the meat more tender and flavorful. Breed makes little difference in flavor in both the taste testing I've done across our four breeds (Yorkshire, Berkshire, Large Black and Tamworth) and this is born out in the scientific literature I've read. Breed sets conformation and to some degree marbling. Age and calories also sets marbling. Feed sets flavor.