best drug to overdose and kill yourself?

UK answer.

Three months ago you asked a question about how to carry out parasuicide [a suicidal gesture without the intent to die]. If you are actually suicidal this time you need to call 999 or better still walk into A&E and you'll be assessed by the mental health crisis team. Otherwise call the Samaritans to talk through how you're feeling or make an appointment to see your GP to discuss medication and talking therapies. Parasuicide is a dangerous game and I've lost 2 friends that way. Mental health professionals can tell the difference between parasuicide and a real suicide attempt and they don't reward parasuicide with a hospital admission as it encourages behavioural problems.

Obviously with all the arguing about lethal over dose it seems even lethal drugs are not very good. I suggest you call 911 and talk to someone before you do something that might hurt.

no

There isn't one, most poisoning attempts fail. All different drugs all have a bad habit of not killing you, or killing you a couple of weeks later, or permanently damaging you but you live.