Most tungsten alloy darts indicate the percentage of tungsten content; this varies from anything from 80-97% tungsten. Generally speaking the higher the tungsten content the thinner the dart for the given weight. Thin darts help with grouping and a greater chance of hitting that elusive 180! Dart weight, shape & design are all personal choice and that is why we can now see a large range of weights and designs. There are no better darts as such as each thrower will have his own preference.
The barrel forms the main part of a dart. When we are speaking of "buying a (new) dart" we mean getting a new dart barrel. Nowadays barrels are available in almost every possible shape, with a wide selection of knurl or rings on the surface for a comfortable and safe grip.
The main barrel shapes are:
We could supply tungsten alloy darts barrels by various sizes and contents, popularly 70~90% tungsten is used for making both soft-tip and steel tip barrels, ranging from 14~18 gram and 18~40 gram respectively, and sometimes, 65% and 95~97% tungsten are offered following customers' specific request.
Billets are actually tungsten cylinder rods after surface grinding by center-less processing, and their weights are varied from different tungsten contents and rod sizes.
It's not possible to say which one is the "right" one. Based on a very theoretical reasoning the long cylindrical barrel could be the best of them because it is the slimmest and allows the tightest grouping. Good for 140s and 180s. But this reasoning doesn't help you when you are not able to handle such a barrel. Selecting the right shape is a matter of personal preference. Choose what feels comfortable for you. This is much more important than an abstract or "academic" reasoning about theoretical advantages or disadvantages of some shapes.