Daylilies.org is a great place to start.. Link below to general, practical explanations of the parts, ploidy and other information on Hemerocallis.

Daylily explanations

I'm not sure what you mean by rating a daylily. Some daylilies do win awards for excellence in different areas such as fragrance or popularity in one or all parts of the country. These award winners are listed at the web site for the American Hemerocallis Society.

The most important rating is the one you make. Do you like big flowers, lots of flowers, unusual flowers? Red ones or white ones or ones with contrasting eyes? The best way to discover daylilies that do well where you live and that you like is to join a local daylily club and go on a summer tour of gardens. Bring a notebook and write down the names of the plants that you like.

If you are far from a daylily club, you could take a vacation to one of the regional daylily meetings that includes a bus tour of many large daylily gardens. Choose an area with the same cold/heat of your home - if you live in Canada, Florida may not be the place to go to scout out plants that will like living in your yard! On the tour you'll see thousands of different daylilies. Some will stand out. If they are too expensive for you to buy now, wait a couple of years because the price will come down. And many fantastic cultivars that were introduced between 2000 and 2005 are already very affordable (and some of my favorite daylilies are much older than that!).
Suzanne

thank you Suzanne, only few are growing Daylilies in Pakistan and most have started a few years ago, but it is catching up.