Void is my favourite (voidwalker ftw), but kinetic is the most useful most often and I'll explain why. Void, arc, and solar all do bonus damage to 1 elemental shield but are resisted by the other 2. And each one does normal damage to non-shield opponents. So not including burn damage on weeklies/nightfalls, each of these 3 elements have a strength against 1 element, week against 2, and neutral on 1. Kinetic damage on the other hand does the same neutral damage to all 4 enemy types. It does not have bonus damage to any, but is not weak against any either. This means it does the same damage to all enemies. Therefore kinetic is the most versatile and the only element that you can use on any enemy without fear of seeing 'resist' flying out from an enemy's body.