Dragon. Without a doubt and without hesitation. I probably would have said that before I narrated Jane Austen's Dragons & The Minstrel's Song, but if I ever had any doubts about it, narrating those books & the reactions that I've received from listeners in particular to my "dragon voices" would tip the 'scales' that way. (teeheehee)
I remember a dragon I drew when I was a boy & took painstaking care to do scales along the whole thing. I now have multiple children who could eviscerate my youthful efforts of draconic drawing.
I'm not sure what the first book was that I read with a dragon. My oldest 2 siblings were dwarves in a production of The Hobbit when I was only 5 or so. My early memory probably enhances whatever effects were used to create the legendary Smaug. I do remember Eustace turning into a dragon in The Voyage of the Dawn Treader. I read some excerpts from Anne McCaffery's Dragonriders of Pern series probably when I was about 10 and that may have been my first literary exposure to "noble dragons," but I glommed onto it & haven't let go since.
Western literature and tradition has tended to lean toward the more destructive, evil & violent portrayal of dragons historically until about the 20th century, whereas Eastern tradition and literature has always portrayed them as more magical, benevolent & lucky. I've loved both Maria's & Jenelle's portrayal of dragons in their books because it kind of embraces both.
Maria Grace's website in particular has some wonderful information on the history of dragon lore that she's uncovered in researching for her books. She also has a facebook group dedicated in particular to the Blue Order described in her books. I'm honored to be at least the voice of some intense creatures.
Honorable mention - The Qanska from Manta's Gift by Timothy Zahn. Like all things Zahn, its a great book with some wonderfully unique concepts.
Any particular creature You'd like to turn into from what you've read?