The Cobalt cape has to be the coolest, the design is original and eye catching. The exclusivity of it adds to the coolness factor.
Minecon 2016 cape. This may be a bit biased because I went to this Minecon and it's the only cape I own, however the Enderman face feels nice on it and not out of place (like the Iron Golem to me on the 2015 cape). It's also very simplistic.
Realms Mapmaker cape. I enjoy rustic areas of nature (guessable from my profile artwork), so the design is very appealing to me.
Scrolls and Minecon 2012 come in close after these