Good flags:
*Original US flag
*Russian/Dutch flags, white/blue/red is a really natural flag IMO, French is okay too... gotta admit
*UK flag, just stylish
*Japan's, simple but kinda poetic and a nice was to represent the nation, despite its complexity
*Brazil's is nice. Has the motto, too.
*Saudi Arabia's. I mean GODDAMN A FUCKING SABRE! can't beat that. Plus some badass wise motto, too, in teh arabics.
*Israel. I don't agree with the country at all but its flag is nice.
*EU flag, nice and simple, again I don't like it
*Canada. Similar, but better than, Japan's.
*Soviet flag. Represents their nations ideas, however badly perverted they became in practice.
I think that covers most of the good ones.