It really depends on a number of factors. Type and age of duck. Whole or parted.
I have muscovy and the absolute best, I think, is to sear the duck breast and finish it in the oven. Muscovy breasts are huge though but more beefy than other ducks.
Legs, I've slow cooked and smoked, both good.
I've roasted also, time intensive but looks nice for a holiday.
Be careful with seasoning. I use just salt and pepper. Some seasonings just don't work.