Which of the three mutually exclusive Clang sanitizers should I default to?

Clang has a number of sanitizers that enable runtime checks for questionable behavior. Unfortunately, they can't all be enabled at once.

It is not possible to combine more than one of the -fsanitize=address, -fsanitize=thread, and -fsanitize=memory checkers in the same program.

To make things worse, each of those three seems too useful to leave out. AddressSanitizer checks for memory errors, ThreadSanitizer checks for race conditions and MemorySanitizer checks for uninitialized reads. I'm worried about all of those things!

Obviously, if I have a hunch about where a bug lies, I can choose a sanitizer according to that. But what if I don't? Going further, what if I want to use the sanitizers as a preventative tool rather than a diagnostic one, to point out bugs that I didn't even know about?

In other words, given that I'm not looking for anything in particular, which sanitizer should I compile with by default? Am I just expected to compile and test the whole program three times, once for each sanitizer?

As you pointed out, sanitizers are typically mutually exclusive (you can combine only Asan+UBsan+Lsan, via -fsanitize=address,undefined,leak, maybe also add Isan via -fsanitize=...,integer if your program does not contain intentional unsigned overflows) so the only way to ensure complete coverage is to do separate QA runs with each of them (which implies rebuilding SW for every run). BTW doing yet another run with Valgrind is also recommended.

Using Asan in production has two aspects. On one hand common experience is that some bugs can only be detected in production so you do want to occasionally run sanitized builds there, to increase test coverage [*]. On the other hand Asan has been reported to increase attack surface in some cases (see e.g. this oss-security report) so using it as hardening solution (to prevent bugs rather than detect them) has been discouraged.

_{[*] As a side note, Asan developers also strongly suggest using fuzzing to increase coverage (see e.g. Cppcon15 and CppCon17 talks).}

_{[**] See Asan FAQ for ways to make AddressSanitizer more strict (look for "aggressive diagnostics")}