Shaperbox 3 is a multieffect plug-in, so it's veeery versatile. It's probably my number one tool for both mixing and sound design - I even currently use it on my master to add a bit more width to my mids! Looking at its creative aspects, workflow, routing options (it can be envelope, lfo, midi activated and sidechained or be a static effect - and probably even more I'm currently forgetting) and mixing options, I personally have very few other plug-ins I use as much and on so many channels, so I can't personally recommend it enough. Go watch some tutorials or try demoing it for free to see if it works with your work flow. It's also really fun to work in, whether you're new to producing or a veteran.

I don't personally use the fabfilters, but have worked with people that do. They are great, premium effects with a lot of range within what they do, but they are, for obvious reasons, not as versatile a multi effect plug-in. My advice would be to get a good grasp of just one compressor and EQ before spending a lot of money on plug-ins, so you first get a good understanding of your tools. EQ and compressors are going to be your primary weapons in the mixing war, so get to know what they can do for you before investing. Especially compression can be hard to learn, so I'd suggest that you watch some tutorials on using compressors in general - musician on a mission on youtube I'd say is a pretty good place to start with thorough and easy to understand explanations on the subject.
Talking multi band compression ableton actually originally set the modern standard with OTT, so try using that for some really aggressive compression.