I don't know if it's the best thing I grill/smoke, but what my friends always raved about was my seafood, especially fish and oysters. I've had many people that didn't like fish or oysters not only eat them, but asked when I was doing it again. When I lived on the coast we used to harvest our own oysters, so they'd always be really fresh.
Really simple technique, just take the whole oyster, unshucked and place them on the grill. As they cook they steam in their own juices, the shells will pop open on their own and you know they're done, if they don't open, just throw it away. On the side, I have a pan with melted butter, fresh minced garlic, fresh lemon juice and some cajun spices. Once the shells pop open, just take off the grill, pull the top shell off, spoon some of the melted butter mixture on the oyster and top with some pico de gallo, then a couple shakes of Tabasco, Louisiana Hot sauce, Cholula or whatever hot sauce you like. Really simple, but really good. I've had several people that didn't like oysters, that would gobble 'em up.
Fish was really simple as well, my favs were redfish and flounder. I used the whole fish, gutted and scaled, de-head the flounder, but leave the head on the redfish. Place the fish on a sheet of foil, slather the outside and cavity of the fish with butter, minced garlic, cajun spice, then add a bunch of sliced onions, bell peppers, sliced tomatoes, and some minced jalapeno. Fold the foil packet up tight and place on a grill, right next to hot coals, on top of coals if they're not too hot. It doesn't take long to cook, use a fork, thin stick, pretty much whatever you have on hand to test doneness, stick it through the thickest part of the fish and when it goes in easy, the fish is done. Fish always comes out moist. Redfish cheeks are mighty tasty, so there would always be a fight stabbing at the cheeks....lol
