The old saying goes, “If you don’t know where you’re going, it doesn’t matter how you get there.” The same is true for DNA testing. A better question than “What is the best DNA test?” is “what are my goals for doing a DNA test?” There are a lot of testing companies out there, but I want to focus on the Big 4: Ancestry, FamilyTreeDNA, 23&me and MyHeritage.
The generic answer that is usually given is to test at Ancestry, then do a transfer (copy your DNA file) over to FamilyTreeDNA, MyHeritage and Gedmatch. If you were an adoptee searching for a birth family (or vice versa), this would be the best way to maximize your exposure -basically, you’re leaving breadcrumbs around for people to find you. I would say in this case, if you can swing it, also test at 23&me to fully cover all your bases. You can opt to do just the ancestry testing, which is cheaper than doing the ancestry and health testing.
Are you interested in ethnicity testing? I wrote a post here about how ethnicity estimates are calculated. I don’t know if there is one company that is better than another for a specific ethnicity, although here are some good posts to read for Native American DNA (which may not even show up in your DNA, even if it is in your family tree) and African DNA. You may also be interested to know which testing companies are available in which countries, because if you’re an American looking for your German ancestors it’s probably helpful to know that Ancestry doesn’t currently offer tests in Germany.
If you had questions on your paternal line (father’s father’s father etc), Y DNA testing can be helpful for those with a suitable candidate to test. For men, this can be themselves. For women, a brother, father, father’s brother, or father’s brother’s son will carry the Y DNA of their paternal line. At the moment, FamilyTreeDNA is the only one of the big companies that does this kind of testing and matching, although 23&me does give haplogroups. You can start with a Y-37 test, and later upgrade without having to submit another sample.
Is there ever a reason for doing an mtDNA test? The Y chromosome mutates more often than mtDNA does, which makes mtDNA better at going back thousands of years to find our ancient origins, but less good at figuring out matches in a genealogical timeframe. The only use I can think of is to compare haplogroups (if you are comparing two women and you want to see if they are descended from the same ancestral female) or if you’re interested in your haplogroup as a marker of a particular ethnicity -for example, having an N haplogroup means your mother’s mother’s mother (etc) was of Native origin. Full mtDNA tests are done at FamilyTree DNA and again, 23&me will give your haplogroup.
Finally, if you are interested in doing a DNA test for health reasons (although I’m sceptical of the value in that), currently 23&me is the only one of the big companies to offer that service.
If you’ve already done a test and are wondering who else to get tested, the same question applies. What is your goal? The generic answer is to a) make sure you have someone on both your maternal and paternal side to help you sort through your matches b) test the eldest generations first. This would mean your testing priority is grandparents, great-aunts/uncles, parents, aunts/uncles, then cousins (further than first if you know of any). I don’t think there’s any benefit to testing your children since they have less of your DNA than you do, and if you’re interested in their other parent’s side you can always test the other parent or their relatives in the same priority order.