I think Butch breasts are really, really hot and sexy. I also really love breasts in general and find myself mesmerized by Femme breasts as well, though I am not really sexually attracted to them. Kind of more of an appreciation kind of thing.
I have been partnered with more FTMs than Butch women, so I am used to flat chests and find them very sexy as well.
I don't have a one size fits all kind of thing going when it comes to attraction and what I find turns me on with my partners' bodies. (Which is not to say there is anything wrong with having strong and particular preferences.)
Boundaries and are really important and doing something to a person they don't like just is not sexy. I have had folks on the transmasculine spectrum let me touch and fondle their chests prior to top surgeries and I found it freaking HAWT and have been with folks who either already had top surgery or wanted that area ignored and that was fine as well.
Short answer: I love it all and nothing is really a deal breaker for me!