I know this will sound rather cheap, but it really is whatever monster or creature that's in my current book. I get to know them, and for that time it is my fascination above all others. In one of my books, it was a tarrasque, and for a time that was my favorite. In another, there weren't any monsters and I didn't think much about mythical creatures. In my WIP, there's a lindwurm. I've used that one before, unpublished.
To come at it from the opposite pole, I don't think much of dragons in general. I do think about particular dragons. So, Temeraire was interesting at first but paled. Smaug has always been a favorite. Ones I actively disliked shall remain nameless, mainly because they made so little impression I don't remember them. IOW, I love well-written dragons and find badly written ones to be deadly ... dull.
To come at it from the opposite pole, I don't think much of dragons in general. I do think about particular dragons. So, Temeraire was interesting at first but paled. Smaug has always been a favorite. Ones I actively disliked shall remain nameless, mainly because they made so little impression I don't remember them. IOW, I love well-written dragons and find badly written ones to be deadly ... dull.