Type O is the most common across the world.
O- is the universal donor.
Most rare is usually B- but will fluctuate to AB- once in a while.
If its a transfusion of the elderly or someone with a compromised immunity OR organ transplant, the person needs to be exact on typing. If your B- you need B-, etc
Also, there is something called CMV negative. Its a common virus that a lot of ppl get. Its part of the herpes virus. Like the flu but once you have and get well you still carry it around with you.