Before I start, I'd like to say that I'm from Denmark.
In regards to flags, I think that there are many cool flags. It's really hard for me to just pick one.
But here goes:
United States of America [http://ec.europa.eu/education/img/flags/usa.gif]
Very visually appealing with a lot of things going on. The blend of colours matches very well. It's also a flag you see quite often (movies, games, news etc.) so it's very recognizable.
Kenya [http://www.ume.tubitak.gov.tr/images/flags/kenya.gif]
A Zulu shield. Seriously. How cool and indigenous is that? Add to that the colours that also matches nicely.
Uganda [http://radiocaptivity.files.wordpress.com/2008/04/uganda-flag.gif]
The colours and the sheer amount of stripes makes this one memorable flag.
Spain [http://www.globalsecurity.org/space/world/spain/images/spain.gif]
The only European country flag that is somewhat different IMO. In fact, most European flags look the same with the same colour scheme. Spain, though still very simple, is at least unique and easily recognizable.
Sri Lanka [http://www.auswaertiges-amt.de/diplo/bilder/Laender/Flaggen/SriLanka,templateId=standard__blob.gif]
Another unique and visually appealing flag. The layout of this flag is very unique amd interesting.