There are a number of signals whose default disposition is to terminate the process. The ultimate termination signal is SIGKILL since it cannot be handled and the process has no choice but to die. This however also means that if you send it, the process is deprived of an opportunity to clean up. Therefore, good manners require to send a signal like SIGTERM that can be handled first and only if the process does not exit after some time send it SIGKILL.
Note that SIGINT and SIGQUIT are not good candidates for arbitrary process termination. Due to the fact that they can be generated from terminal's keyboard, many applications use them for special purposes. For example, python interpreter uses SIGINT to generate
KeyboardInterrupt exception (also in interactive python sessions where it simply returns to the prompt) and JVM uses SIGQUIT to dump stack traces. SIGINT and SIGQUIT do remain effective for most standard command-line utilities like
During system shutdown, most UNIX and Linux systems send SIGTERM to all process, followed by 5 seconds wait, followed by SIGKILL. This is the recommended way to safely shut down an arbitrary process.
Note also that even SIGKILL may not terminate a process stuck in an uninterruptible wait until the process wakes up.