I'd be remiss if I didn't preface this response with a caveat thusly:
I do not condone suicide or suicidal ideation, even. When the issue of suicide is ever broached (which, surprisingly, it frequently is in my offline conversations), I'm always reminded of that pithy but inspiring Charles Bukowski quote:
"Sometimes you climb out of bed in the morning and you think, I'm not going to make it, but you laugh inside — remembering all the times you've felt that way.”
But enough concerned admonition and ado.
Any barbiturate will suffice. One of the most popular drugs for suicide or assisted suicide is phenobarbital. However, I'd personally recommend secobarbital as the beau ideal lethal somnifacient.
In veterinary medicine there's a formulation called Somulose—which contains secobarbital—used to euthanize small-to-medium-sized animals, like horses, dogs, pigs, sheep, etc. in a humane and painless way.
People suffering from terminal illnesses with bleak prognoses (say, pancreatic cancer, Alzheimer's disease, multiple myeloma, and other afflictions) have been known to journey to Mexican veterinary pharmacies to purchase phenobarbital to preclude a painful, protracted death in lieu of a peaceful drug-induced respiratory depression.
1.)
A synopsis of pharmacological suicide options
2.)
http://www.patientsrightscouncil.org/site/problems-assisted-suicide/]An article basically tacitly corroborating my foregoing choice of barbiturates as a nonpareil pharmacological suicide method