It depends on what you mean by "Brightest". The 4300, 6k, 8k, don't have ANYTHING to do with "Brightness", its more like "Hue" than brightness really.
4000 k is closer to yellow - 8000 k is closer to blue, and so forth.
Its really just a way of describing the effect of heating something, and it radiating light in various hues as it hits certain temperatures....like metal might be red hot, then white hot, then blue, etc....as you heat it up in a forge, etc.
The human eye sees more sharply with yellow than blue for example, a reason shooter's glasses are yellow/amber most of the time, etc.
Yellow glares less for the human eye, blue glares more.
For example, if its raining, the blue light seems to vanish before you, very little reflects back to your eyes in a useful fashion...as the blue is dispersed by the water/vapor, etc.
This is why for example fog lights are also yellow/amber typically, as they will provide more USEFUL light, rain or shine.
So, on a clear night, you can perceive a higher color temperature (Degrees Kelvin) as being "Brighter", and, under good conditions, this will be true until the kelvins go up past "Clear" to "Blue". As they kelvins rise into that frequency, your eye has fewer receptors that can see the bluer light, and the USEFUL light is gradually diminished.
It also generally takes more POWER to emit higher frequencies, and less to produce longer wavelengths...so watt for watt, you get more "light" at the yellow/red end of the spectrum than at the blue/violet end.
When we use ultraviolet detection equipment at work, it takes a lot more watts to "illuminate" the target than if we are using infrared, etc.
This is also why most laser pointers used a RED dot, instead of other color, as we can SEE red better, and, it takes less POWER to send that red laser beam out than if it were blue, etc...
...so a red laser, watt for watt, is more "Powerful" than a blue one.
The human eye can only perceive a very narrow band of the spectrum, ranging from a wavelength of about 380 nanometers (Bordering on ultraviolet), up to a longer wavelength of ~ 740 nm or so, bordering on infrared.
We typically "See Best" at ~ 550 nm, which, is actually about as "Yellow" a light as you'd get.
Its because the 3 types of cone cells (Receptors) in your eye each respond better to different parts of the range of light wavelengths...with the overall best
overlap being ~ 550 nm (Yellow).
So, yellow light is actually best from a performance standpoint....and any variation from yellow is a compromise made towards fashion.
My 55 watt HID's are about 6k, (White looking) for the record, and perspective.
I installed them into the stock headlights, and they glare into other drivers, including the policeman's, eyes, so as to blind them.
This makes it difficult for the policeman to give chase, as he is unable to see well for a while after getting blasted, etc.
To reduce the numbers of angry blind policeman wanting to discuss my lighting situation with me on an urgent and insistent basis, I looked at WHY it was glaring, and saw that the HID capsules did not emit light from the same point WITHIN the headlight as the OEM Halogens did.
Essentially, the headlight is a reflector housing, with the reflectors designed to focus the beam of light in a useful PATTERN on the road, with a cut-off so as to NOT blind the local constabulary, etc.
When the HID capsule emits the light from a different point relative to the reflectors (In this case, further from the reflectors/closer to the headlight face), it changes the PATTERN that is then reflected back out of the headlight.
It flipped the pattern UPSIDE DOWN.
That meant my light pattern cut off was protecting the worms and root systems of nearby vegetation from glare, albeit was sending light meant for the road into the eyes/mirrors of those unfortunate enough to be in front of me, or even along side the front of the truck, etc.
I solved this problem by applying a strip of silver duct tape to the BOTTOM of each headlight face...which, as the beam pattern was UPSIDE DOWN, created a cut-off at the TOP of the beam pattern.
So, now, if I am behind or driving towards a trooper, etc, my low beams, despite emitting from my admittedly higher headlights (About the level of their heads to start with), do not inspire them to want to discuss my lighting system on an urgent basis.
I did get pulled over by a cop to the side of me, as "Your head lights reflecting off my side view mirror were melting my retinas" or something like that, seemed a topic that he wanted to discuss immediately with me.
So, in retrospect, if doing it over again, I'd replace the OEM headlights with projector lamps designed to work with HID capsules, as a unit (Matched), so that I could use more of the light to see with, and less of it to heat up the back of silver duct tape/tempers of blinded motorists.
