Dragons, because they're majestic, powerful and everything they do is an event. A dragon flies by out hunting for food, it's an event. Running dragons in combat is certainly fun and fighting against them, well, that's pretty good too. Adventurers scramble around their entire careers picking up +5 uberswords and orbs of killing every mutha in the room, becoming legends, and then an ancient dragon with ten names crawls out of its hole, dusts itself off and slaughters them. Dragons are great!
If I had to pick a type of dragon I'd say blue, because they can burrow, imitate voices and lightning is one of the neatest breath types. Some of the others like gas and antiethical energy (?) are not that dramatic, and fire feels too generic.