I would like to know from you guys, what criteria do you use to select the perfect (green) coffee bean for roasting purposes?
Your definition of "perfect" may be different than mine. Some people prefer specific types of coffee beans, such as Brazilian, Sumatran, Mexican, Kenyan, etc. I assume that is NOT what you mean, since that is based on your own personal taste.
I assume you are asking where you should buy your greens. The easy answer is to purchase high quality green coffee beans. This means that you should purchase from someone who you trust. You basically have to rely on other people's advice, and see if you agree with their recommendations.
After doing my research, I joined the green coffee buying club, where I buy most of my beans. I have have also purchased from (and recommend) Sweet Marias. Their web site is a treasure trove of information on coffee, including green coffee beans and coffee roasting.
If you are hand selecting than you might refer to AA standards for the chaff, etc.
Also discoloration from the median color of the other beans can lead to irregular roasting intervals (if you have a mix of colors they may "crack " at different ranges of time). Similarly you should prefer the beans to be approximately the same size.