Q: What are the best filters for mastering?
Well I would say that in any artistic pursuit there is no such thing as 'best'. It's all down to personal taste, and what pleases your client or market.
So by this logic there is no such thing as the best filter for mastering or the best EQ.
There is one exception to this in general in music and recording... That is where you want to copy a certain sound. So if a producer asks you to get the same bass sound as he has heard on his favorite Lady Gaga song, you will be off to a good start if you can find out which instrument and setting were actually used.
Back to mastering...
Suppose you have perfected your mix to the maximum and feel that it's the right time to start the mastering process.
You will reach for your favorite mastering plug-in obviously. Well that's what most people do.
But what you could consider doing is applying the various processes involved in mastering 'by hand', rather than just slapping a plug-in on the buss.
One favorite mastering process is multi-band compression. It's powerful, and you can really screw things up with it. But it can really help you fine-tune your master, if you are careful and know what you are doing.
So you could insert a multi-band compressor.
Or you could do it 'by hand'.
What you can do is use auxiliary sends to send copies of your mix to four new stereo auxiliary tracks. (I would really like to call them channels, which is what they are, but I'm using the language of Pro Tools for this example.)
Arrange things so that only these four new tracks go to the master fader, and none of your original mix.
Now, in each track insert a filter. Not an EQ - a filter, or filters. You might find your filters as part of your EQ plug-in. This is fine, just don't use the EQ part.
Set the filters to split up the frequency band into four, so one track handles the bass, one track the lower mids, one track the upper mids and the final track the high frequencies.
So for instance in the LF track, you could set a high-cut filter at 200 Hz. For the low-mid track you would set a low-cut filter at the same frequency (200 Hz) and a high-cut filter at say 1000 Hz.
You can choose a filter slope, but all the slopes should be the same. Start with 18 dB/octave.
If you can do that, the rest of the filter settings should be obvious.
Now when you mix the four tracks together then it should sound pretty similar to your original mix. I say similar rather than identical because it depends on precisely how the filters are designed whether everything will mix together 100% correctly. But since the mastering process changes the audio significantly anyway, we won't worry about that for now.
Next, insert a compressor into each track after the filter(s).
Guess what? You now have a multi-band compressor to play with, and you made it by hand.
So go on and play with it. Experiment with the crossover frequencies and slopes, particularly experiment with compression ratios, thresholds and everything else compressors can do. Use the faders to balance the levels of the four bands of frequencies.
I can guarantee that if you spend a few hours experimenting with this, everything you do with purpose-designed multi-band compressors will be very much better in the future, to the benefit of your mastering skills.
Back to the original question. Really it isn't a matter of having the best equipment or software. You need decent equipment and software, but after that it's all down to how you use it.