Lead is not a vitamin.
There is no 'best' vitamin. Most are essential to health and the lack of them can lead to disease and even death.
In real life, all vitamins and trace minerals are essential for reproduction. In proper balance. In Howrse, the answer is Vitamin A
Vitamin A is essential for eyesight.
Vitamin A.
Yes, vitamin B1 is essential for its synthesis.
yes
Vitamin A
Vitamin K
Vision and bones.
Vitamin C.
calcium is essential for teeth and bones
Vitamin A is essential for breeding stallions.