The UK Office of National Statistics also publishes data on all deaths from poisoning (i.e. intentional and unintentional death). In that table (see Drug poisoning in England & Wales), class A drugs like heroin/morphine, methadone and cocaine come highest, followed by paracetamol, anti-depressants and insomnia medications. The data from the US (see Drug poisoning in the US) also features a similar top four.
So the statistics give a clear indication of which drugs can be used to achieve a drug induced death, although they don't actually show, by method, what the percentage success rate of each method is. If the overall ratio of success with a drug induced suicide is 40 to 1 against, it is probably likely that even the drugs that appear highest on the table still have a fair number of unsuccessful attempts.