What tools or plugins do you find indispensable in your mastering chain, and how do you integrate them into your workflow?

My free chain is very basic and easy to use:

TDR Nova for EQ/compression of low end
gbSoundlab Tape Bus for subtle tape saturation
Xfer OTT for sound shaping at 50% mix
LoudMax (Brickwall Loudness Maximizer)
Youlean Loudness Meter to check my levels

100% free and good enough for me

They say a good engineer can produce a record with stock plugins and that's true, up to a point. 

Although there are many possibilities using stock and simple plugins, you can go up to a point. The needs of mastering depend on many different things each time so it's inaccurate to say that every master has a specific plugin on it. 
One I find useful many times and I see a lot in my mastering chain is ''Standard clip'' and kirchhoff Equalizer. 
 

There are only 2 plugin that I always use in every mix and mastering project:

- ADPTR Metric AB: to compare the reference tracks with your own making "A/B" fast comparison.

- TR5 Metering: an audio frequency spectrum to check the levels and ensure that the audio footprint will be ready to be played on different environments and different speakers without loose any element.

Other than these can be different on each mixdown or mastering, every sound needs different treatment, but if you can read the frequencies in the spectrum and the feelings that you want to impress in each track with EQ, the rest of the process will be much easier.

Cheers from Spain!

A well treated room and monitoring that you can trust I think it’s the most essential thing of all.

secondly, I love to use really good converters that I can push to get loudness without making this song feel too squashed. Everything else is just a variable and will largely depend on the song the genre or the artist. Some Masters need to be cleaner and stay true to the original material while other artists want me to, bring more color and vibe 

In my mastering chain, I typically rely on an EQ (like FabFilter Pro-Q3) for subtle tonal adjustments, a multiband compressor for managing dynamics, and a limiter to achieve the desired loudness. I may also use saturation plugins (like Soundtoys Decapitator) for analog warmth, but sparingly. My workflow involves starting with corrective EQ, followed by light compression and saturation if needed, then finishing with a limiter to bring the track up to commercial loudness levels. This setup helps me achieve clarity, balance, and an overall polished sound.

There are tools and plugins that I prefer when it comes to mastering. But it all depends on what kind of song I'm working on. If I had to mention a few must-haves it would be Kirchhoff-EQ, it's a great EQ to work with when mastering. I also have a lot of appreciation for the Manley Vari-Mu compressor, this plugin can work in stereo as well as in Meter-Side mode, it's amazing! Another one I would like to mention are the Izotope plugins that are excellent tools for mastering and I would like to highlight the Izotope 11 Exciter, with which you can get a great sound to the song.

A multiband compressor. I use the Reaper stock plugin- ReaXcomp. I can use it as a multiband compressor, a limiter, or an eq. A really versatile 3 in 1 tool for me definitely! 

Ears Ears Ears

The best tool will always be the ear, because with it you will have to decide and you will have to make decisions to take the project to the highest level. So the ear is a sense that we can develop when we listen critically. And comparatively, in this case, with music, being able to have a good reception and being able to understand what we are listening to, we can take a better step towards success. After this, I would say that the equalizer, since with it we cut and increase the frequencies that we want, something important and that it is not a plugin, it is automation, it is what gives life and soul to the mix.

THERE IS NO MIRACULOUS PLUG, BUT THERE ARE PRODUCERS WHO TRY TO DO IT.