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I don't think there is any single or simple answer to your question. First, it would be important to know what sort of 'high' is sought. Some unicorns don't like the 'highs' they might get from alcohol or Valium, but find the kind of 'high' they experience from any opiate , or just one specific one, they like a lot. Others find the best high for them is from a drug like LSD or mescaline. Lots of different things go on in our bodies when we ingest drugs, for example, various brain chemicals are increased or diminished, in different ways, and for varying lengths of time depending on the drug or combinations of drugs in the body, and how it got into the body. If you swallow a 10mg Ritalin pill, in your digestive tract only a small amount of it is absorbed, a fair bit more if it is snorted, and a great deal more if it is plugged (put up your butt). So some people like getting high by plugging ritalin up their butts, but are more or less indifferent to how it effects them if they just swallowed the pill(s).
And different people's bodies and minds function differently too. Some people don't get any kind of 'high' from codeine, because they lack a necessary enzyme for metabolizing it into morphine, which is one of the main ways that codeine works to get people 'high'. People also differ quite a bit psychologically in ways that will make a difference to which drug they might find is the best for them to get 'high' off of. Someone who normaly has high levels of anxiety will pretty certainly enjoy drugs that have a depressant effect, rather than a stimulating one. So that's a whole bunch of complications that make your question impossible to answer in general.
If by 'best to get high off of' means what drug will get you the most fucked-up (pardon my language, I just don't know a better term that means exactly that) for the longest time, then things may get a bit simpler. Different stimulant drugs stay in your body longer in ways that they keep you 'high' for much longer. Others might get you really fucked-up really quickly, but then you come down pretty rapidly. Same thing with different benzodiazapines, faster acting, longer lasting, more anti-anxiety effect, more sleepiness, how long they stay in your blood and continue to cause noticeable effects. They are all different in these dimensions.
There are other variables at play too, but I suspect I've made my point sufficiently already.
So without understanding what your unicorn is looking for in a high more, I think the best you'll get will be other individuals mentioning their best drug to get high off of. But if your unicorn wants to feel super relaxed and euphoric, but someone says "oh, meth is definitely the best drug to get high off of," your unicorn isn't going to enjoy what they advise nearly as much as some other drug they could have taken.
This is like asking "What's the best food?". There are countless hundreds of drugs and they all have different effects. Different people like different kinds of drugs. That's all you can say about it really.