"Well, it depends on what ya mean by efficient. Botulinium toxin, when ingested, is a very deadly poison that causes the nervous system to fail and immense pain at death, but, it doesn't kill ya in a day or three. Tetrodotoxin — again, ingested — can cause a paralysis then death within just six hours. Of course, the well known Cyanide, can be both ingested and inhaled, fairly easy to get your hands on to, can lead to death within minutes.
But yeah, I have all of those, just depends on how you want 'em dead. Though, just to tell ya, I like keeping the Tetrodotoxin in my personal collection."