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These are 5 of my most favourite, go to plugins when it comes to mixing music.
I wouldn't say any particular plugins are essential for getting great sounding mixes, you can absolutely get great mixes with the stock plugins in your DAW, however certain plugins can help you get to that end results faster and easier! And when your mixing, you don't want the tools to get in your way - you need to be able to focus on the music.
I use lots of plugins when mixing, but this selection from Slate Digital and UAD are probably in my top 5 right now!