Trying to list all the things to love about MODX would be futile - there are just too many. But there are a few things about MODX Revolution that make it the Best CMS in the World, IMO. This is not to say Evolution, the classic platform, is any less great. In fact, I know many developers who believe it's still a better choice for most of their applications. If Revo didn't exist, I'd still be happy as a clam developing in Evo. However, it does exist, and here are 3 reasons I thank the MODX Core Team for it every working day of my life...
Package Management » Maybe it's laziness, but pressing a button to install an addon just seems...better - vs copy & paste of various components, don't you think? The packager also allows distribution of templates, custom installs, etc that help take MODX to the next level.
Contexts » More and more, my clients are wanting a "family" of sites, rather than just one website. Is it just me who's seeing this? Before Contexts, this would require another Manager install or domain forwarding and masking - neither are perfect solutions. With Contexts, multiple sites can be run from one Manager interface - and with true domain independence, the rest of the world needn't know they're related unless you want them to ;)
Advanced Installation » Installing the MODX core folder above the web root of your hosting account is a great way to increase security. And with MODX becoming more and more popular, the standard folder names for the Manager and Connectors may become a target for malicious activity. Advanced Installation allows you to customize those. Want to get jiggy with it? Generate a hash key as the folder names :P Less easy to use, but way harder for attackers to find.
What are your favorite features?