Depends on how long I've played it. With some games I prefer harder skills if it's a game I've played a lot. For instance, I beat Duke Nukem Forever on the hardest skill setting for the achievement just because it's one of my favourite games.
That being said it also depends on the game. With some games, especially Bethesda games, the difficulty can be all over the place and something like Normal can be either too easy or Hard can suddenly be too difficult. Thus I often end up switching difficulties in Fallout 4 and Skyrim due to how there's no sweet spot.