3mg Nicotine
3mg is the sub-ohm strength. If you are running a tank at 40W or more, this is almost certainly what you want.
The reason is simple: a sub-ohm coil delivers far more vapour per puff, so a low strength still gives you plenty of nicotine. Put 20mg salts through the same coil and it would be unbearable.
Most 3mg here is high-VG, made for direct-to-lung kits rather than the tight cigarette-like draw of a pod. If you are vaping to stay off cigarettes and using a small pod kit, look at 10mg or 20mg instead.