You have to pick single plant or animal to live off of for the remainder of your life. What would be the best choice in terms of health and longevity? Your selection will be delivered to you fresh but dead every time you need it. You can only eat the tissues that are an integral part of that plant or animal. Stomach contents or parasites living on it do not count.
You animal get this and water to live of of. What is the best choice?
Humans.
Cannabis?
Well, any** one** thing you pick is going to be lacking some vital amino acid. If I was trying to prolong my life as long as possible with one food, I would pick the potato. It has just about everything I would need. I would eventually succumb to malnutrition, but the potato would keep me going the longest.
I’d think any one thing wouldn’t last very long. A single potato would hardly last a day. I would have to go with something much bigger, like a blue whale. One of those would last a long time.
D
If I understand the OP correctly, you get unlimited quantities of whatever it is, but can only pick one thing.
I believe the proper answer is the peanut. It contains the overwhelming majority of the nutrients essential for life, in a very rough approximation of the quantities needed to sustain life. It’s by no means perfect – you will definitely eventually develop a Vitamin C deficiency, and also B-12 – but it comes the closest to being the single ideal foodstuff. (You will, however, have to consume something, perhaps some other part of the peanut plant, for bulk, to assure proper colon function.)
I don’t know what you were trying to say in your sentence above: “You animal get this and water to live of of.” Can I choose a genetically-engineered plant?
I think I’d go with soybeans. It has an awful lot of protein, and I’d imagine some other stuff that’s important.
I should’ve included a smiley, I thought I was making a funny.
The thread title seemed to suggest just one of something, though. You see, if you had a “single dead” something, it would only last so long and you’d eventually consume it all and starve, regardless of how healthy it was. I was racking my brain to think of a really, really big dead plant or animal. I’d figure a dead whale would last long enough for me to die of something other than starvation before it’s gone. 
Shramp. Because if you have shramp you can make barbecue shramp, shramp creole, shramp cocktail, shramp gumbo…
Any source of plant protein will be critically deficient in at least one amino acid, so you’ll eventually develop some kind of protein deficiency if you live exclusively off that one plant. Plants will also be almost certainly deficient in at least some vitamins and minerals.
The more closely the biochemistry of your food source resembles your own, the more viable that food source is as an exclusive one. (I mean this broadly; let’s exclude primates, including humans, as a possibility because there are medical as well as deep ethical issues with that.) Any mammal should be close enough; you could pick one you know is tasty, like cows, or one that’s really big, like an elephants or blue whales, to reduce the frequency of shipments. You’ll get protein with roughly the same amino acid profile that your proteins require; you’ll get all the vitamins and minerals that mammals need. There are a few ways that primates differ biochemically from other mammals, but they shouldn’t pose a major problem. (An example: most mammals can synthesize vitamin C.)
Some human populations have survived for long periods of time with a very limited variety of food sources; the Inuit/Eskimo are probably the most familiar to North Americans.
Hence my answer above.
Heck, if you’re going to drag in ethical considerations, you’ll take all the fun out of it. 
Human breast milk.
Yep. Which makes me think raw whale meat may be the way to go in response to the OP. (Yum!)