The problem with knight designs is that it's very easy to turn out something with not much detail on it. The usual idea of Western knight-style armor is wearable, flat metal plate, so knight designs tend to converge toward a generic man-in-a-huge-tin-can kind of design. Thus, the first thing I looked for in deciding on which entry gets my vote is that the design had detailing on it that made it unique without being messy.
With the above in mind, MeanBeard's Immortal Knight II gets my vote. There are a couple of things going on in this design that make it unique. The most obvious one is the helmet. I never liked that helmet, but combining it with the plume, the Feathers of Strength, and the helmet ornament turn it into something completely different. That part of the plume also adds to the facial hair only sweetens the deal. The other thing that makes this design unique is the use of the Xiba 1P top. Using it as a tabard-like part over the rubber bodysuit was a stroke of genius. Finally, using the phoenix sticker on the patterned bodysuit to make the chestplate and chain mail was a great move. I'd have liked to see some detailing on the shoulder armor and the leg armor, if possible, but I still have no problem giving this design my vote.
I'd also like to bring recognition to the helmets in each of chobek's entries. Each of the helmets brings out the unique character of their designs. I was especially impressed by Ducain's bird helmet and Dragon lady's twin dragon helmet.