ive killed an abnormal amount of dragons in my option for my level and ive noticed it was just dragon at first, then blood dragon then, snow dragon, now im killing dragons by there name any option
As you level you will run into more and more powerful dragons, culminating in Ancient dragons. Dragon>Blood Dragon>Frost Dragon>Elder Dragon>Ancient Dragon (from easiest to most difficult).
Named / unique dragons are pretty much quest related or involved with the main quest plot in some way.
Well I am level 44 and fighting Elder Dragons only.. They are a little easy.. They die in like 6 hits.. And that is like 10 seconds if they go on the ground..