I checked the analyses done by consumerlabs.org and all brands passed. Buy what is cheap. 4000 IU per day of Vitamin D3 should suffice and it is extremely cheap.
Most people have low vitamin D. People with darker skin and more fat mass tend to have the lowest values.
Getting 20-30 min of sun (avoid face) per day can provide 2000-4000 IU of vitamin D but most of us do not get enough sun. Fortified milk and orange juice do not have enough.