Hallo
Hi / Hello
Casual, works almost anywhere
Guten Tag
Good day / Hello
Formal, general daytime greeting
Grüß Gott
Hello
Regional — southern Germany, Austria
Moin
Hi
Casual, northern Germany, any time of day
Servus
Hi / Bye
Regional — Bavaria, Austria; works both ways
Wie geht's?
How's it going?
Casual follow-up to a greeting
Wie geht es Ihnen?
How are you? (formal)
Polite follow-up with strangers