Rust dragons are easily my second favorite dragon.
I like dragons in two categories: alien superintelligences and ravening dumb monsters. Too often, they're some watered down middle ground (white dragons have intelligences higher than most wizards, spells, and speak a bunch of languages, and yet are described as feral beasts). While ravening dumb monsters are awesome though, it really cuts into the true impact of dragons. Dragons as super-villain, almost godlike beings who would as soon snack on the puny, dumb humans as consort with them. Many can polymorph into humanoid shapes as well, meaning there can be awesome reveals as to who the bad guy is.
Another factor in the horror of dragons is their destructive potential. Aside from the fact that they're giant reptiles with teeth and claws and tails that can level houses, they have breath weapons, and spells. Some even have multiple breath weapons. I love the fact that a dragon can level a party, but most of my favorite dragons have ways of mutilating a party aside from draining their hit points. Rust dragons destroy their stuff, shadow dragons drain levels, and my favorite dragons, brine dragons, drain Strength and bring pain.
Look at this majestic beast!
While technically an aquatic dragon, my favorite place for brine dragons are inland salt lakes. One of my favorite villains I've ever run was a brine dragon by the name of Riozzo. My group collaboratively designed a game world to run a collaboratively DMed series of games in. The world, Everith, was a gas giant with floating islands of land. There were airships, and it was a space-opera-ey type game except without the space part and with 100% more D&D (technically Pathfinder). The second of our games was on a moon called Batham. Batham touched the atmosphere of Everith and had its own life forms and ecology. One of the main points of history was a war between the Inevitables (Law) and Proteans (Chaos). Riozzo was subcontracted by the forces of law, and was the guardian of their towers on the edge of a salt lake crater.
Now, that was thousands of years before our campaign. The war between law and chaos subsided and died off. Riozzo devoted himself to the studies of the universe (physics, time control, astronomy, and alchemy), awaiting the time he would be called back to the war and setting things in motion in his own way. From beyond the stars, he brought many strange lifeforms, experimenting on them, and, once he had learned all he cared to from them, set them as guardians around his caldera home. He was the stately figure that loosed his Akata hounds to feed on the town I mentioned in my favorite NPC post, and eventually became something of a looming presence in that campaign. Riozzo has also appeared in other campaign worlds of mine. I justify it by him plane hopping as immensely powerful beings are wont to do.
Honestly, the fact that he surpassed the simple stats of an ancient wyrm brine dragon is the reason he's my favorite, but salt as a simultaneously corrosive and purifying agent played for some important imagery for him and my way of justifying his actions.