This isn't a 'best primer' tip, but I find myself wondering how different the primer products out there really are.
...because I actually use automotive primers fairly frequently (cringe), just for the simple fact that a little dough goes a long way.
And on a side note, I recently finished an imperial star destroyer kit, and used a somewhat light grey automotive primer from Home Hardware or something, and wow- it's friggin' amazing how well it turned out; it looks exactly like it does in Episode IV. (with no additional paint, etc. needed)