Well, taking the question literally, I'd have to say the Australian cone-fish. Note only does the barb kill instantaneously, but within around 30 seconds all traces of the poison have disintegrated from the body, making the cause of death untraceable. Otherwise, I'd slip a funnelweb spider into the guy's shoe/jeans. Wouldn't arouse attention because they often make their way into human habitation like shoes and clothes, and are possibly THE most aggressive spider towards primates - they'll chase you across a room if angry, and there's high a proportion of encounters that lead to bites (they're fast, with a biting mechanism that can piece shoes and jeans). The Sydney and tree varieties have been known to kill a human within 15 minutes, though everywhere in Sydney carries anti-venom, which has effectively prevented deaths since then. Nonetheless, ultra agressive spider that chases you, bites through shoes and jeans, and can kill within 15 minutes - pretty good poison if you want to kill someone, hey?
If we're talking self-intoxication I'd go either fine red wine, fine port, or sweet but simple cocktails (butterscotch snaps and milk, white/black russians, bundy and coke - that or straight single mark scotch if there's a good one available. I'm in my 30s, so the idea of drinking cheap crap I left behind many years ago. Otherwise, I'd say benzodiazipines - maybe xanax or rivotril. Medically cleared for both, and must NEVER combine them with alcohol, but used sensibly they're a nice relaxant if you want to curl up with a book and bliss out on a Sunday night. Don't want to get addicted to those things though - you can dy from benzo addiction, so it's worse than heroin addiction insofar as drug abuse goes. Mind you, kiddies there's nothing really worth 'abusing' about it - they're just calmatives. Being calm, relaxing and resting feels nice. That's it. No 'high' to speak off. Folks who get addicted are either heroin addicts using it to increase the effects, amphetamine addicts who need it to chill out and sleep, or idiots who'll take anything that mildly affects sobriety. For most people, you wouldn't want to 'abuse' it unless you actually needed it - in which case you should be able to find a doctor, where so long as you're honest and try the other options first, they'll prescribe it for you. Just need to make sure you're completely honest about your usage so you don't end up a waste of life.