Depends- a LOT. In reality, for actual models, real porn, date material and such... I'd say practical, perky, with puffy nipples (PPP for short). Whatever easier to live with, and stays up well with age. Better to pick a partner that looks good long-term. If there's discomfort, pain or health problems involved, it becomes a turn-off.
Now, in FICTION, with stuff like 3D models and drawn art? - That's a whole other deal. Like heaven and hell different! If you remove the issue of a real person being involved and open the doors to pure fetish fuel, we don't need to keep any sense of right and wrong anymore. Without reality to keep it down, imagination has no limits. Morality can go straight out the window!
Then I often prefer them obscene, wildly out of proportion, impossible to hide away or ignore. We're talking head-sized, can't see your feet, need custom-made clothing big here. At least!
I want them to dominate her life. The less she can keep up professional appearances and get taken seriously, the better. I want them to get in the way constantly, gravity having little to no power to hold them down, slightest movements being able to set them into jiggling motion, until she can barely even do desk work anymore. Bonus points for dystopias where it's the default size for everyone.
In hindsight, I used to have much simpler kinks, but I've explored my limits too far now to settle for anything sensible when it comes to fiction anymore.
