The pain level of a heroin overdose would depend on the level of overdose. In cases of a large overdose, the experience would be relatively painless as the person would be unconscious before the organs began shutting down. In the case of a slight overdose, it could be very painful.
Overdosing is not painful. It is difficult to say which overdose would be most pleasurable, since I think that's really what you're asking, because, much like death, those who succeed are not available for comment.
Slowing slipping away while you sleep would probably be the least painful. Heroin can cause seizures, heart problems, vomiting and eventually death. However, there is a good chance you will live and be permanently damaged.