I wear a neck knife daily, as a last ditch backup weapon, not for utility. For actual cutting, I have various folders I usually can get to when needed.
I have a few things I like in a neck knife. First and foremost is rust resistance! BG-42 and even S30V have both fallen, rather quickly, to my acidic sweat, so, for me, I won't wear a neck knife, unless it is made of titanium, stellite or talonite, or, I guess, ceramic too.
The second thing I want, is light weight, if it's heavy and flopping around inside my shirt, that's no good, so, Ti is especially good for that reason.
Lastly, a flat profile is nice, for the same reason as weight, makes it conceal better and doesn't flop around and get in my way.
For all these reasons, the best neck knife I own is my Mick Strider custom XL Ti SLCC NM. It sports a nice, long (capable) 4.75" blade, which, due to its Nightmare grind, can stab or slash effectively, but, because it's titanium, it literally feels, weight-wise, like it's a piece of paper, it is amazingly light for its size, feels like it barely weighs more than the para-cord that it hangs from, and yet, it is fairly thick, and would certainly be strong enough for most self-defense encounters. It is VERY sharp, on all of its
3 edges

. It also lays perfectly flat, and of course, Ti is rust proof. It satisfies all my requirements.
I occasionally wear others, and in a way, I kind of feel like, some men, and of course women, are into jewelry, I am not, but, it almost feels like my neck knives are my jewelry, the difference being, my wife's heart pendant can't save our lives if need be and is anything but tactical.