What key nutrients are required for daily energy?

Uh, I don't know where to begin. This is like, the entire field of nutrition.

Vitamin D does not need to be obtained from the diet, it is primarily produced in the skin (from cholesterol, which you don't need to eat; your liver produces all you need from any food) after sun exposure. The amount of sun exposure required depends on the time of year, time of day, cloud cover, and amount of skin exposed. VitD can be obtained in the diet but pretty much only from fish and food that has been supplemented. I assume you are now taking high-dose supplements, because the amount in a typical multivitamin isn't enough.

That said, fatigue is associated with iron deficiency, and if you menstruate there is a ~25% chance you aren't eating enough to have optimum levels - look into a blood test. (If you don't menstruate, you're probably fine, but a blood test can't hurt). 

I am like 2 months away from having an undergraduate degree in nutrition, and I'm studying in Australia, so I recommend this website to you: https://www.nrv.gov.au/

It outlines all the nutrients (from carbohydrates/protein/fat to the vitamins and minerals) and gives the recommended dietary intakes for each. It describes the scientific basis for each, with citations. 

This is a calculator you can use to get a 'personalised' recommendation for each nutrient: https://www.nrv.gov.au/nutrients-energy-calc 

EDIT: Thinking about it a bit, is the question you actually meant to ask "what are common nutritional deficiencies that I might not know about"? That depends on many things, but Vitamin D would be #1 on my list. 

Well, at some level, most of them. But which may be limiting your own energy is going to vary from person to person. Vitamin D is a common one, I take it too. For me, taking fish oil, zinc, and a B complex (with food, they make me nauseous on an empty stomach, and no mega-doses) are also helpful. So is proper hydration. For me, that means at least 100 oz of fluids a day, and I find it helpful if it has a splash of citrus or other juice in it - the little bit of sugar seems to matter, ditto for it being helpful for me to eat high water content fruits and vegetables.

When, how often, and how much at a time you eat matters too. Some people do well with small meals more often. I have more energy eating keto but have a hard time keeping to it. I also do better intermittent fasting (anywhere from 16:8 to OMAD) with my main meal in mid-afternoon or later with (usually) little to no refined carbs and refined oils, and I've been doing that for a couple of years.

You say you're constantly tired. Does it vary throughout the day? Is it worse at specific times, or times relative to meals, or in response to different types of activities? Timing food and hydration based on mental, physical, and emotional activity can matter a lot.

Beyond nutrition, lots of medications can cause drowsiness in some people (I have allergies and every antihistamine causes me serious mental and energy issues; the least bad for me is Allegra and even then I can only take it once or twice a week at most).

How is your air quality at home/work/wherever? Ambient noise levels? Light levels? (I find brighter indoor light helpful, but conversely, I found that being in the sun makes me tired, and started wearing a sun hat and darker sunglasses when spending a lot of time outside). These can have both physical and mental effects on energy.

Since you mentioned a healthy brain, keep in mind that it's also very easy for mental health to affect energy levels in ways you might not immediately notice. There are the well known ones like chronic stress, depression (it took me 10 years to realize I was depressed; I spent 5 of them thinking I had a sleep disorder, and was on modafinil for 4 of those), and so on, but also things that vary quite a lot from person to person. For example, I find strong emotions leave me feeling drained, even good ones.

And there are genetic factors: how are the rest of your family's energy levels? I grew up thinking it was normal to not be able to stay awake throughout a movie even when you're enjoying it and it's mid-day, or to fall asleep mid-conversation after a meal, because most of my immediate family has those issues like I do.

In addition to the 23andMe recommendation from ChristianKI, you might want to consider trying Viome, an at-home blood and gut microbiome panel. My wife did it (not for tiredness), and the recommendations were very specific, tried to be explanatory in a way that wouldn't overwhelm most people, and (with some analysis and research and background knowledge) pointed towards a single underlying cause.

My impression of the state of things is there are a few things almost everyone (or typical Americans, at least) should be taking.

1. Vitamin D (defaulting to 2000 iu until you get a blood test to check)
2. Omega-3 (absolute minimum 1g, ideally 2, and I take 4 or 5 but for joint pain)
3. Psyllium Husk (or equivalent)
4. Some kind of magnesium, small dose, especially at night.

Sunlight is great for Vitamin D but impractical for many months in the year. Sometimes Vitamin D with K2 in the same pill. Everything else is situational. 

A good starting place is to use an app called Cronometer, log all the food you eat for a week, and identify obvious gaps in certain nutrients. If you can fix these with diet changes, that's ideal. If you can't then consider a supplement.

There's a huge rathole if you look into methylation and B vitamin stuff, it's really almost too deep. Easy to get lost in. The TLDR is some people have huge genetic differences. It might be worth trying something like L-Methylfolate, but this is also in the scope of supplements that can be harmful if you take too much. You can export you 23andMe to give you an estimate for whether something here might help.

Some people have genetic mutations that mean they need 10x more of one Vitamin than other people. If you want to make sure that you are actually getting enough of all Vitamins, doing a 23andMe test can be helpful to see whether you have any mutations that change your Vitamin needs.

Otherwise, there's the general result that broad-spectrum multivitamins don't seem to be healthy in the normal population (there are some results that might be helpful for some poor people). 

This suggests that you likely get some negative effects if you consume too much of many nutrients.