My character can burn calories and turn it into a beam of light a few centimeters in diameter with nearly 100% efficiency. What wavelength would be best as an antipersonel ability, calorie for calorie? The range will be under 100 yards, and some enemies will be wearing full plate armor. He can adjust the output, but would burning around 1000 calories in one shot be enough to kill one enemy with a headshot?
Probably microwave 1 to 10 cm
There's a reason we use this to cook food, water is a good receiver of energy at this wavelength, and humans are mostly water. So if you want to cook someone's brain, that might be your best bet. It has the range you want too, microwave communications span 1 km or so, but it'll be less for a high energy weapon. The key is that you have to do it in a burst, not a sustained ray.
FYI, yes I know that technically the classification of microwaves is larger than this, but this is the range that is most commonly used in microwave ovens.
I agree with Mathaddict for people naked or wearing regular clothes, however the metallic armor might reflect some of the MW radiation.
Use gamma rays, those will go better through armor. It also adds radiation poisoning, which is everlasting - so you know that if the bugger survives they're less likely to be alive or in shape to go for revenge.
By definition, 1 calorie is (roughly) the energy needed to warm 1 gram of water by 1°C
So, if the 1000 cal are focused on 1g of water, it will reach 1000°c and simply explode. If it were spread over 100g, it would raise temperature by 10°C and could cause minor burns
BUT as @AlexP explains, calorie ≠ Calorie == kilocalorie == calorie (in nutritional terms)
Assuming you are talking about food calories, that is a million real calories and that's enough to raise the temperature of 1kg of water by 1000°C. The vapor explosion might be similar to a hand grenade.
About the wave length, I'd suggest you use infra-red or visible light. Those wave-lengths are immediately absorbed by the skin so the whole energy of your beam will be absorbed by a very low quantity of matter (that's the scenario where you heat only 1g). This will cause a vapor explosion with the maximum detonation velocity localized DIRECTLY on the skin, sending a terrible shockwave inside the body.
When it comes to armor, it would totally depend on what kind of armor. Is it medieval metallic armor? A ceramic modern thing? Was it designed to protect the wearer from your magical ray of death?
Basically, I'd say that medieval plate armor (much padding with a layer of shiny steel) would offer good protection.