[junk88]

What is the best eq? And do they effect music quality?

 

[ProtegeManiac]

What is the best eq?

Depends on what how much control you require to correct a particular response issue.

And do they effect music quality?

Yes.

Some of them can add noise, like Neutron Music Player on some devices around 2014/5.

Or you can screw up the settings.

 

[silent-circuit]

If you're using PC as a source Equalizer APO is a good place to start. Not the easiest thing to work with, but powerful.