Three chins? Well I only have two, but I go for the V-neck
I’m a v neck person also. I like the way boat necks look but I feel like I’m suffocating when I wear them. Weird.
@BunsenBurner, it’s not merely a matter of chest size, but of, um, placement. I have a lot of space between my collarbone and where my breasts begin (shortwaisted, too–yeah, this is not the easiest body to dress), so even though I wear a DD cup, I’d have to wear a push-up bra to achieve cleavage in most clothing. On the other hand, D has very little space–i.e., her cleavage is high–so any v-neck other than the most shallow one will be more revealing than one typically wants with daywear. Stuff that looks fine on most people will be totally inappropriate on her. She does wear a lot of tank tops under shirts to handle the issue. We humans come is a big variety of shapes!
Mostly V-neck but scoop neck is OK too. Anything so long as it isn’t a crew neck, which I think makes my shoulders look too broad.
MommaJ, of course, we all do. I think that dark (mostly vertical print) wrap-like tops with deep v-necks with a contrast color insert (tank underneath) would minimize the “round object on top of two spheres” effect all too known to me. Of course, if she has an elongated or oval face, that would not be an issue. 
Your D needs to find a better bra style, too. 3-part structured cup instead of the molded - I can’t believe they make molded cups and lace cups in large sizes. That’s why the need for all the uncomfortable wires to keep the bra “functional.”
We are all different, aren’t we. I’ve never found underwire bras to be uncomfortable. In fact, I find them more comfortable if they have the wire. YMMV.
VH, I think the brands you and I wear perfected the concept of underwire. 
(or should we say underplastic) but we also pay $$. My comment referred to cheapo stuff sold by Victoria Secret etc.
I’m trying to think of a clever way to ask this, but, what the hell, I’ll just ask: how do you know what brand of underwear Very Happy wears, Bunsen Burner? That’s a surprising thing to know about an online friend! (No snark here, just genuine curiosity!)
Lol, I might be mistaken, but I think it was her who mentioned Wolford and Marie Jo in one of the threads! Heck, if we can discuss COLONOSCOPY, why not bras?!
BTW, I am shopping for a swim suit and came across this:
http://m.herroom.com/prima-donna-026-1816-deauville-full-busted-spacer-contour-bra.shtml
http://m.herroom.com/prima-donna-014-1350-twist-rebelle-three-part-full-cup-bra.shtml
This style takes care of the unneeded cleavage.
Somewhere in CC land a male is reading this thread and wondering how cleavage could ever be “unneeded”!
I have to say I thought I wore pricey bras, but those Prima Donna models are astonishingly expensive even at the sale price.