This site has a listing of all the DL fare buckets, but it's in alphabetical order.

There might be a simpler way to figure out the fare rankings, but the one I'm aware of is to go to DL.com. In the 'Book a Trip' box, click on "Full Search Options". In the lower right, there's a pull-down menu for "Fare Class". When you click on the arrow, you'll see a list of the fare buckets in order from low to high (T, U, L, K, Q, H,.....).