I've absolutely fallen in love with Foundry VTT because of it's openness, the excellent UI, and the incredible community that has been built up around it.
The openness of the system allows anyone with programming skills to create a system (which represents an RPG ruleset) or a module (which represents additional features like automation, animation, pretty much anything really). Even if you don't have the skills to develop something, it's quite simple for anyone to work within the frameworks that are provided to customise and add your own homebrew "stuff".
It makes it so much easier for me to do so much more in my online games - for example, I've been adding music, ambient sounds and sound effects to recent sessions with ease, something that used to be quite painful and generally required using a separate app or Discord bot. Now I put a little bit of thought into what I want to use during a session, source and add the files to Foundry and now as I switch between scenes I can have a playlist start up automatically or start firing off sound effects with a single click whenever required. Recently I used this during a viking funeral to sound a horn and play a traditional Norse funeral song and my players absolutely loved it - now I'm looking for sound effects that I can use when I get around to running the Alien RPG.
The only thing that a lot of people don't like is the fact that there is no official content or marketplace, so in most cases if you want something you need to add it yourself, although systems for RPGS that have an SRD do tend to have that content available. There are community modules for importing content across from Roll20 and DnDBeyond which does offset this issue, and personally I haven't had any problems with manually adding the content that I need for my games - it's easy and really just a matter of how much time you have available.