In terms of simplicity, Alpine Linux is unpeered. Alpine is the only Linux distribution that fits in my head. The pieces from which it is built from are simple, easily understood, and few in number, and I can usually predict how it will behave in production. The software choices, such as musl libc, are highly appreciated in this respect as well, lending a greater degree of simplicity to the system as a whole.
Source: Praise for Alpine Linux, an article by Drew DeVault.