What's the best tiling window manager in 2021?

Well, it's still i3 [1].

[1] — https://i3wm.org/

For me it's bspwm. It's closer to i3 in simplicity, but has some dynamic tiling functionality that "cooler" WMs do. It's very modular, the documentation is great and you can get started in just a few minutes.

"power users" or people who are willing to get their feet wet with code can check out XMonad (Haskell), AwesomeWM(Lua), and Qtile(Python) as well. DWM (C) is a popular choice too nowadays and is rather nice, but I can't bring myself to care about terribly documented software that justifies it by saying "we'd like our user base to be small and elitist."

I always enjoyed dwm [1] the most, but I have since switched to a wayland environment and with that I am now using sway [2].

[1]: https://dwm.suckless.org/

[2]: https://swaywm.org/

i3 has been a favorite window manager for a number of years, though I recently started using EXWM and greatly enjoy the translation layer (simulation-keys) that allows Emacs key bindings to be used in other applications.

Why specify “in 2021”? You worried someone’s gonna hip you to the bet tiling window manager of 2037 instead?

been very satisfied with i3. haven't tried any others, unless tmux counts.