A recent review demonstrated that out of 1 million test files, Windows Defender missed 3 files.
However, the review took into account, how fast it scanned, the general aesthetics of the GUI interface, among other things and its total percentage rating as a result dropped to 84%.
In my personal opinion I ignore these reviews as they are paid for by someone who wants you to buy their product, or use their so called free utility and to me these reviews are generally a load of bollocks as a result.
As an analogy, its like asking one of these television doctors which drug is best for X condition, when really you should be asking them, which Drug Company is paying them to host the TV show.