As you can probably guess from the image I gave this post, my favorite is the flag of Bhutan. The yellow & orange create a distinctive & satisfying background, and the dragon in the center makes for a strong focal point that doesn't clash with the background due to its simple color palette.
A close second would probably be the flag of South Africa. The pattern and colors here are just really nice, dunno what else to say.
Probably not that unique, but mine is the Canadian flag. The red and white with the maple leaf is just so pleasant to look at! Also, due to the simple two color design, they have implemented it beautifully in their governmental branding and stuff. Simply gorgeous!
They really did knock it out of the park with that one. Simple, readable, and iconic. It's got a symbol that can be used to evoke the nation. The color scheme isn't overused by other flags. 10/10.