It really depends on what your interested in. If you want to work at a donor center it would help to go to a fellowship that has one in house.
Medical College of Wisconsin (Versiti Blood Center of Wisconsin) is very, very highly regarded.
UT Southwestern MC- Dr. Sarode is an expert in coagulation and management of sickle cell patients. If you're interested in those areas. (Mayo sends fellows to UTSW for education on management of sickle cell patients)
John Hopkin is also very strong, big apheresis service, and very well known in the field. JHU developed the PLEX low dose IVIG protocol to clear DSAs in transplant patients.
Cedar Sinai is well known for great blood bank/transfusion training. Joe Chaffin aka "Blood Bank Guy" (if you don't know google his site). I also believe they have a donor center at their facility which is a plus.
NIH has the person who identified Hep C, its a great program for research oriented individuals.
Mayo and UPenn are great programs especially for cellular therapy, they have separate fellowships for it.
Emory, WashU, Yale, NYBC, and UNM are all good places as well.
I'm sure there are many other places as well.