Why, every single cousin if you are a real Gene/Genea-Geek. If you are not a Gene/Genea-Geek and you are just getting started the answer is as simple as:
What do you want to know?
To find information about your Mothers direct maternal line, you would test a male or female family member who is descended from any of your mothers, mothers – going back as far as your would like to go. They would get a Mitochondrial (mtDNA) test. This is a very deep test which can follow a line back for a very long time, it “is passed down by the mother unchanged, to all her children, both male and female.”(ISOGG – Mitochondrial DNA Tests)
For your fathers line, you can test your brother, your father, and any male cousins who share your LNAB surname. They can take a yDNA test. This is another very deep test which can follow a line back a great distance. “The Y chromosome, like the patrilineal surname, passes down virtually unchanged from father to son.” (ISOGG – yDNA Chromosome DNA Tests)
For other relatively close cousins, that are not in your direct maternal or paternal line, you can test anyone who matches you through your genealogical research. They can take an Autosomal (auDNA/atDNA) test. This is not a deep ancestor test and “can be used to confirm relationships with a high level of accuracy for parent/child relationships and all relationships up to the second cousin level. For all relationships other than parent/child relationships additional contextual and genealogical information is required to confirm the nature of the relationship.”(ISOGG –Accuracy of tests)
If you are interested in knowing your ethnic genetic make-up then you would only test yourself. This would give you your specific Haplo Group, “a genetic population group of people who share a common ancestor on the patrilineal or matrilineal line.”(ISOGG – Haplogroup)
Where to test? You want to have your DNA and the others (the people you want to test) DNA in the biggest DNA Gene Pool(s) as possible. From a recent class on DNA on Triangulation with Kitty Munson Cooper she suggested starting your testing with Ancestry for atDNA, then transferring your results over to FamilyTree DNA and upgrading to other tests for mtDNA or yDNA from there. This way you have your DNA in two big gene pools right off the bat (this as of march 2016).
Now run out and find all the cousins in your genealogical paperwork and talk them into doing a DNA test!