Simple time delay
If the target takes medication, preferably hard capsules, it makes it really easy. Pick any number (or combination) of fast-acting (or slow, I suppose) poisons, fill a capsule, and place it in their medications/vitamins. Hard capsules will take a few minutes to several hours to digest, and can be made fairly precisely. Replacing the contents of capsules was used in 1982 to carry out the Chicago Tylenol murders.
In small enough quantities, poisons are undetectable by taste or smell. Toxic metals have the ability to bioaccumulate, or build up in the body; lead and mercury are well known, but there are any number of toxic metals. Unlike some poisons, since metals are elements, they can't be destroyed, which means that they can be delivered through smoke (poisoned tobacco or candles), food, liquid, chairs, even soap - anything the victim comes in contact with. Unfortunately (for the poisoner, at least), most symptoms of metal toxicity are recognizable and often treatable before death, meaning that unless you use a really big dose, chances are, your victim is going to live to make it to the hospital. The best option would be to slowly administer small amounts over the period of a few months or weeks, waiting for you victim to begin exhibiting symptoms; early symptoms are usually ignored, but if you know what you're looking for, you can spot them easily. Once the first symptoms occur, follow up with a moderate dose. The larger dose, coupled with the built-up smaller doses, will surely kill your target.
Poisons are useful because they are cheap, easy to procure, easy to administer, and at least some of them are hard to trace. However, certain particles are deadly in doses as small as a few atoms - radioactive materials. While some radioactive materials take hours or days to kill someone, once ingested, if you really want to be sure of death, use some polonium. Polonium may be hard to come by, but it is effective. With a large enough dose, death would occur within seconds. Followed by the deaths of everyone in the house, block, and possibly city. I would suggest wrapping the dose in a lead sheath and placing that in a capsule. And hire someone else to administer it.
Death by Boom
As long as we're talking about elements, a capsule containing Caesium would be sure to make an impression. Caesium metal reacts violently with water, producing hydrogen gas, then igniting the gas in an explosion. Not only that, but a dose of 4.1 micrograms of Caesium-137 is enough to kill a dog; a dose of twice that would surely be enough to kill a human, and would be such a small dose that there is no chance it would be detected in food.
There are a lot of poisons that would fit your requirements, enough that you can choose exactly how you want your victim to die, even down to a precise time. Do you want to be sneaky or bold? Does timing matter enough to be precise to the second, the hour, or the week? Messy, or clean? Quiet, or violent? Fast death, or lingering pain? Undetectable poison, or a brash warning to others?