What is the best Linux distribution for old computers?

Arch has strict reasoning behind what goes into the official repositories accessible by pacman. As a result, many other projects end up in the Arch User Repository (AUR). Effectively using Arch, and getting easy access to all of the available software, means either manually downloading and installing from the AUR, or installing an AUR helper - these work along side the main package manager (Pacman), but are a bit less standardized or heavily supported.