Earlier quoted context omitted.
For open source libc? musl, glibc, dietlibc, bionic are the only I know of. The supported targets vary widely among these, though.
There is the freebsd libc; they run their own.
freebsd -> darwin, openbsd, netbsd and dragonfly bsd. openbsd and netbsd like to add their own stuff on top of it also.
The Open Watcom libc and bionic are also superior. bionic is getting better and better recently.
musl is in some parts better, in some parts worse. Same for Embarcadero C++ libc.
Worse than glibc are only: the two MS libc's, msvcrt and ulibc (which is getting on par), the wine libc, newlib (arguably with dietlibc the worst), uClibc, minilibc.