Most disgusting: Delvers. Rocklike skin, covered in corrosive slime (highly acidic so tummy no likey), eats rocks (the meat will be atrociously tasting and textured to humans and very likely toxic), it was sapient and just as smart as you were when you were alive so there's a bit of existential disgust, BUT, crucially, you can actually eat them. If you want a less obscure answer, Carrion Crawler. Tentacled abomination(gross), big green insect with lots of insect meat and guts(gross), probably tastes like insanity and the way the color green looks when you're high on mushrooms, stinks of rotten meat at all times, even when cooked.
Most delicious: Dragon, Red or White Dragon specifically. Dragon meat is quite literally supernaturally good for you, and while taste and texture depend on what color or metal or gem or other type of dragon, Red and White would be the closest to common meats humans eat, and can be prepared in any way you can prepare normal beef for the flesh. Dragon organs also have many beneficial properties that don't involve food and some that do, in addition to dragon bones also being useful for weapons and armor and making the magiscientifically best possible soup broth for recovering from a cold.
Basically, an adventurer out for glory and money, a foodie out for the supreme meat, a blacksmith out for weapon material, and a dying old man out for an immortal replacement heart are a valid D&D party for doing nothing other than killing dragons.
