Yellow is definitely the easiest, but it's also the Times New Roman of flag colors.
I would love to hear about some unique flags. It seems like an easy way to stand-out. But I guess flags take a beating. You would need to rotate them out every few months to stay looking sharp.
The flag sticks at Philly Cricket and Baltusrol are a work of art (cloth and wood).
Lots of clubs have spent lots of money on this front trying to be distinctive. Most of the wooden flagsticks I've seen do not really function that well, but rich guys think they are the bees' knees.
The most unusual flags I've seen [for better or worse, as tastes vary]:
Wind socks at The Challenge at Manele
Double pennants at LACC
Wicker baskets at Merion [not as rare in 1920, you see them in old photos of SFGC and some other courses]
Very short flagsticks at Secession
Those are the ones that come to mind right away, I'm sure I've missed some.
My favorite ever was on a course I saw in Zambia last year. I could see it from a distance but was not sure what I was looking at. When I got up close, I found that it was some kind of tree branch or plant, with a single, very large leaf near the top of it for a flag. But it was just on the one hole!