I’ve been mixing feature films and sound design for the last 15 years so plug-ins are my daily life. Here are some of my highlights:
Izotope Neutron Advanced covers almost every mixing situation. Neutron dynamic EQ is excellent - it’s an EQ, a de-esser, a pseudo multiband dynamics tool… But Sculptor is useful for acoustic and electronic sources. If you don’t mind the interface, Fabfilter Pro Q is also great, so are DMG Audio’s offerings. EQuality in particular. Waves Abbey Road stuff is good.
Reverbs, Delays and Multi-FX:
Valhalla DSP as has been said here multiple times. Exponential Audio reverbs are excellent as is their Excalibur sound mangling tool (currently on sale I think). Soundtoys - especially Echoboy, Crystalizer and PrimalTap. Eventide have some cool effects too.
TAL Juno and SH-101 emulations, Arturia V Collection are excellent. If you’re into the CS-80 sound, Memorymoon ME80 is an often overlooked emulation. Native Instruments have some powerful, more modern synths, but I find a lot of NI stuff takes too much time to set up and thus kills the vibe for me.
Waves TG Mastering Chain, Elysia Alpha Channel, Shadow Hills Compressor, bx_digital and Izotope Ozone are among the best.
However! Don’t overlook those built in stock plugins that come with your DAW. It is very much possible to create excellent sounding mixes with the tools you already have. Think about it in a modular way - if your stock reverb sounds too sterile, put a chorus before it, and put saturation after, then follow up with a subtle phaser and feed that into a delay. You can design some incredible, unique effects using the most basic building blocks!
Don’t overthink it!