Alternatives for XQuartz?
There are really no strictly better alternatives.
XQuartz is usually the best option for running an X server on a Mac.
If you're running older OS X versions, you might prefer X11.app from Apple.
If you're using older Mac hardware with PowerPC CPUs, you can also run XFree86 in the Darwin/PowerPC port (note that Xorg.org is originally a forked XFree86).
If you simply looking for alternatives - you could also take a look at WeirdX. It is old, it is slow and doesn't have as many features as Xorg - but it is an alternative:
http://www.jcraft.com/weirdx/
Besides that - there's always the option of running for example Linux in a virtual machine on your Mac, and then run your X server in there (for example Xorg).
Also you could look at alternative ways of connecting to your Linux desktop. For example you could use Xpra:
https://xpra.org
There's a macOS client - but there's also a HTML5 client for accessing the desktop through a browser.
Another alternative would be to use VNC instead of the X protocol.
UPDATE January 2021: An XQuartz 2.8 beta has just been released with support for Apple Silicon!