This all depends on the sort of tank you have. For most mouth-to-lung kits, which you would use like a cigarette, you want to use a 50/50 liquid. These allow you to have higher nicotine and will also work perfectly with the coils in your kit. If you have a more powerful, straight-to-lung kit, you would likely prefer a high VG liquid like a 70/30. These thicker liquids work better with these bigger coils, produce more cloud and better flavour.