AviSynth vs AviSynth+ vs VapourSynth: which one should I choose?
As a summary of the differences:
AviSynth

Outdated and no longer recommended for use
Only available in 32bit – not able to use 64bit plugins/filters
No multi-threading (unless you install AviSynth-MT, another fork, which is prone to errors and issues and no updates over last 4+ years)
Older and only supports 8bit video
Lacks support for some color spaces (YV16, YV24 etc.)
Latest release over 4 years ago (May 2016)

AviSynth+

Has both a 32bit and 64bit version, although the 32bit version apparently has some issues
Supports multi-threading which can significantly improve rendering performance for some functions (similar to AviSynth-MT, another fork, but AviSynth+’s appears more stable)
Be aware that using the 64bit version will require all plugins and filters to also be 64bit – and not all of them have 64bit versions. There are some filters that are 64bit only, however, which means you’re able to use those.
QTGMC runs faster in multi-threaded mode, and also apparently in 64bit mode
AviSynth+ specific documentation is limited, but most of the great AviSynth wiki also applies to AviSynth+ and many pages now have notes mentioning any specific instructions for working with AviSynth+.
Also runs on Linux, which is useful when you need to run it on a server
Regularly updated, with latest release Jun 2020

Note: be sure to get a recent version of AviSynth+ from https://github.com/AviSynth/AviSynthPlus/releases. There was a pinterf fork and it has been merged back to upstream.
Or you can use the Universal AviSynth Installer which allows you to easily switch between the different versions of AviSynth, AviSynth-MT, and AviSynth+ (32bit and 64bit).
VapourSynth

Rewritten to use Python, a much more powerful and flexible scripting language
Multi-threaded
Supports generalized colorspaces whereby new or custom colorspaces can be specified at runtime
Also runs on Linux, which is useful when you need to run it on a server
Documentation isn't as detailed as AviSynth (it's not a community wiki)
Regularly updated, with latest release Aug 2020