Some game designs fit well with difficulty settings. Some games have implemented this bad: for example most Bethesda games are just increasing enemy stats (which result in million-hits-to-kill enemies on the hardest difficulty).
Gameplay quality in such games often tightly-connected with chosen difficulty. How to choose it right? Previously i had a simple rule: hardest difficulty for everything with no exceptions. Later i’ve decided to start with “normal”, then increase it each playthrough.
Normal difficulty is often the best-designed. During development, when there’s lack of time (and this is always the case :-)), designed can’t put as much time in designing other difficulty settings as they put into default setting - which is often “normal”.