I am going to do this in two parts I will do a good dragon, and a bad dragon. Because I think they are different, and should be talked about.
So let’s start with the good one. I am not sure about other editions, but in 1st edition the copper dragon would try and talk to people and if they did not talk back, the dragon would burry that person to his or her neck and then force them to talk. I don’t know why I love this, but for this reason this is my favorite good dragon.
I think that through all the years, my favorite evil dragon is the Blue one. I don’t really know why, I just like it. That being said I like that in 2nd there was the book of dragons, in the book we learn of the yellow dragon. I know that the great thing about D&D is that I could have turned any dragon in to a Yellow one, but I still like having it right in front of me.
My favorite monster over all comes from the 1st edition Monster Manual II, on page 86 starts the Modron. They are funny looking things. My favorite one is on page 88 the Tridrone. And there it is all my monster and dragons.