If you want to study at a German university, you need one of these three certificates: TestDaF, DSH or "telc Deutsch C1 Hochschule". Different universities and fields of study require different test results. If you know, which university and course you want to sign up for, you can check which certificate will be accepted and which result you need. Here is a discussion about German language tests for admission to a university.
To find a job in Germany the B2 exam is most suitable. In some cases a B1 certificate can be enough.
For a settlement permit or to acquire German citizenship you need a B1 certificate. There are three major B1 tests. See the overview of B1 tests to make your choice.
For a visa to join your spouse (Familienzusammenführung) passing the A1 certificate is enough. A1 is also the requirement for a blue card.