The important characteristics of the garment are about the corners, size, material, and how open the sides are (see below). The choice of neckline would be governed by what you're going to wear it under, same as for any other undershirt.
Laws of Tzitzit from Chabad gives the following requirements for the tallit katan. Some of the stringencies here are not universal (see comments); note, however, that these stringent requirements say nothing about the shape of the neck-hole, the subject of this question:
It must be a garment for the body and not just the head (such as a shawl). But unlike an ordinary shirt, the four corners must include two in the front and two in the back.
If the front and the back of the garment are connected at the sides, a clear majority must still be disconnected. According to the Alter Rebbe the tallit katan should be completely open on both sides and not be attached with any stitches or clasps. Certainly the garment should not have sleeves. The garment must be of a woven material, and preferably, it should be made of wool and be white.