The best one I've found, so far, is the Spyderco SPOT. I prefer the one with thin black micarta scales, because it also comes with VG0-10 steel, instead of AUS 6.
I've tried quite a few. Another one I like is the HAK (HideAwayKnife). The thing about the SPOT, is it's basically a skeleton knife & the way it's designed with the finger hole, it allows just about a full hand grip, but is only 4 3/4" OAL. that's small.
I like the finger hole, because it's easy to index & isn't going to come out of your hands easily (If you need your neck knife, you're pretty much in deep doo-doo). Also, the tube that runs along the top of the sheath does keep the knife from flip-flopping under your shirt, thereby making it hard to grab the knife properly.
The HAK would be a close second, IMHO. But, I found that it'd flip around & just seemed not be as good. I guess it's because we/I use knives all the time, that the SPOT is more natural because you still have a handle to hold.