i3
The tiling window manager most X11 users tried first, with a readable config file and windows arranged in a tree.
Michael Stapelberg started i3 in 2009 after giving up on wmii's codebase. Where dwm has fixed layouts, i3 models the screen as a tree of containers that the user splits horizontally or vertically, stacks, or tabs. Configuration is a plain text file, and an IPC interface lets external scripts query and move windows.
The low barrier made i3 the default pick for anyone curious about tiling who had no wish to learn Haskell or C. When Wayland took over, Drew DeVault wrote Sway, which reads i3's config format almost line for line — for many users the migration cost a logout and little else.