I tend to favor breasts anywhere from an A to a C (pretty much my outside limit). "If it's bigger than a handful, you're risking a sprained tongue". I have a personal phobia against MM ones, as well. Frankly, they may be great to look at (usually under clothes), but once you get your paws on them, they're just no fun to play with (in my personal experience, anyway). They have a notable tendency to be rock hard, no "give" to them, and frankly, I always get the impression that they were really intended for a museum (look but don't touch). Totally agree with the aforementioned perkiness factor, though...that is really key.
It really comes down to "what works for her frame" when I am trying to decide if I like them or not (or, really, more likely "how much I like them", as "not liking" them isn't a super-realistic option).
I dunno, sometimes the lady radiates such beauty as a whole, that it just doesn't matter, but that's coming from a woman's perspective. To me, there is nothing more sexy than a confident girl that glows from the inside out.