What is the best vitamin D supplement?

Dietary supplements can contain either vitamin D₂ or vitamin D₃. Both forms are well absorbed in the gut and both forms raise serum 25(OH)D levels, so both are beneficial to maintain adequate vitamin D levels. However, vitamin D₃ is thought to be slightly more beneficial, as most evidence indicates that vitamin D₃ increases serum 25(OH)D levels to a greater extent and maintains these higher levels longer than vitamin D₂.