* Formal: `Goedemorgen` (Good morning), `Goedemiddag` (Good afternoon), `Goedenavond` (Good evening). These are all perfectly acceptable and polite greetings in formal settings. You could also use `Dag` (Day) which is neutral but polite.
* Informal: `Hoi` (Hi), `Hey` (Hey), `Dag` (Day - also used informally), `Hee` (Hey).
While `Dag` can be used both formally and informally, the context makes it clear which is intended. Using `Goedemorgen`, `Goedemiddag`, or `Goedenavond` in an informal setting isn't wrong, it just sounds a bit stiff.