In Java critical sections, what should I synchronize on?

private void doSomething() {
  // thread-safe code
  synchronized(this) {
    // thread-unsafe code
  }
  // thread-safe code
}

public void foo() {
    synchronized (this) {
        // do something thread-safe
    }
}

public synchronized void foo() {
    // do something thread-safe
}

Vector v = getSomeGlobalVector();
synchronized (v) {
    // some thread-safe operation on the vector
}

public class Foo {
    private final Object syncObject = new Object();
    …
}

private Object mutex=new Object();