Answers (2)
Most people have pretty eyes even if they don't realise it; a lot of my brown eyed friends think that theirs are boring and plain, but it's not the colour that makes them beautiful, it's the light behind them.
There isn't one that I consider superior to all others - they're your inheritance from your ancestors. Also consider that people can (and often do) have multiple colours and shades in their eyes, so just choosing one makes things a bit more boring.
Are you asking because you're writing a character or just out of interest? If the first, then take into consideration their ethnicity and personality. If you choose to give them an unsusal one, then think about how this might effect their life, eg. How/Why did they get their colour? Does it set them apart from their friends or family? Have they ever been discriminated against because of them? Are they proud of their differences or do they try and hide them? Can they help them in any way?
If the latter, then think about the people that you find beautiful. They probably don't all have the same eye colour - it doesn't define their attractiveness. Also consider that eyelashes, eye shape and placement on the face are all important factors when appraising beauty.
Hope this helps :)