If a group of people live in a closed loop environment, with all energy coming from a reactor. What crops would you grow if you only care about keeping people fed for the absolute minimum amount of power consumed?
UV is a poor energy source for agriculture, with enough energy per photon to easily damage living organisms. If you have a source of kilowatts of UV and want to grow crops, you need to convert it to kilowatts of longer-wave light, which can be done quite efficiently with a variety of phosphors. This is in fact how fluorescent lights and most white LEDs work.
Then, once you have light tailored to the plants needs (likely pink or purple in appearance): plants using the C4 pathway are somewhat more efficient at photosynthesis. These include maize, sugar cane, and sorghum, among numerous others: https://en.wikipedia.org/wiki/List_of_C4_plants
You will of course produce large amounts of cellulose as well. With the help of the right microorganisms, you should be able to convert that into carbohydrates with more food value for humans or substances of use in your chemical industry, use it to produce plastics and fibrous materials, or just compost it to produce more growth medium for plants. Likely a combination of these.
It's going to be hard to get more efficient than algae. We're happy (some of us, at least) to put spirulina powder in smoothies, and so forth, so we already eat it.
Algae will be probably the simplest source of nutrients, taking the least energy per gram to grow up.
Unfortunately, I've no idea if it's nutritionally balanced, but I suspect it could be engineered to be. It'll also be able to use any waste heat to speed production, and be possible to grow in a sphere around the light source, maximizing growing surface area.
Microbial electrosynthesis is more energy-efficient than photosynthesis. Engineer microbes to produce nutrients and use electricity from your reactor to power the process.
Most important question to properly answer it is: for how long?
For whatever reason you chose to concentrate on power consumption. This is fine and I can see at least several reasons for doing so, all perfectly legitimate, but I believe you failed to consider several factors, most important of which is that proper diet is needed to keep humans in more or less healthy condition. And it's not constrained to just physical health. For example, if you'd limit your people to corn you'd eventually end up with serotonin deficiency and pellagra, which some studies link to Aztec cannibalism, for example.
So, depending on the time you plan your group to spend in the closed environment, you have some options, but since vegan diet is very hard to get right without really wide food variety available, you may have to revise your assumptions.
If we're talking about 6 months tops then soybeans or mentioned corn. Soybeans are really good in photosynthesis department, but need to be properly fermented (which takes energy). Corn is also a good choice. But you have to add supplements to it, or introduce more plants, especially vegetables and nuts or oily seeds; though not necessarily a lot.
If you are talking long-term, then without either supplements (vitamins, fats and minerals) or really varied foodstuffs (grains, beans, vegetables, fruits and/or - unfortunately - animals), your group will not survive long.
This means you have to either invest seriously in the area for food production or, which is a better way, introduce plants with much better photosynthesis efficiency (be it breakthrough in cross-pollination or selective breeding or genetic engineering), because then you can get 10 times or 100 times higher yields from area, which you can use for diversified heavily crops or still have room for animals (should you choose so).
Obviously upside of any farming area in closed loop environment is a good way to boost air scrubbing of CO2 and water vapour, so this is a good way to offset energy expenditure there. Also, this whole area may be an excellent place to have for improving all-round wellbeing of the crew. Humans do not do well long-term in enclosed spaces, so having an area that can double as recreational is a benefit.
That or a genetically modified analog, as you need to give better details on the level of technology you have available and the amount of space you can afford. If this is a colony ship for example, space, weight and power are your currencies, while for a space station, the space is most valuable.
But I will assume a closes cycle explorer starship, in which case mushrooms would be your primary choice. They grow with little sunlight, can be modified to decompose crew waste and can be grown in artificial mediums. Only problem is that they kind of suck at making oxygen, if you care about that.
Otherwise, Soylent Green
If you do care about oxygen, then algae, reprocessed into edible bars are most efficient. They'd taste pretty alright and you are making oxygen while you do it, so you'd fit food and life support into a single system.
Do remember that I assume you have relatively advanced genetic modification, or otherwise find the perfect kind of edible mushrooms and seaweeds to feed your crew, which might be hard.
For mushrooms, a hydroponic artificial medium combination, possibly along walls of a farming level of the ship, would use reprocessed waste as fertilizer. Otherwise, algae would be grown in well-lit vats of water, with carbon dioxide being bubbled through. If you want to maximize the shielding efficiency, then you'd make the walls of your space ship serve as the water tanks, filled with algae or otherwise, but do note that it would make cleaning them a pain in the rear end.