The Sony EQ's seem to have the least impact on the delivered sound quality and are 'musical'. The rest? Not really worth using IMO if you're going to make a point about sound quality. The core sound quality on the current NW-Axxxx is however I think slightly lower than the iAudio X5, the Creative ZVM or the iPod.
The X5, Zen Vision M, the iPod, etc (including the Monolith) all 'grain up' the sound when EQ is on. Out of that the iPod does it least (but of course it has hopeless presets), the ZVM does it most (and it also distorts when you take the bass boost to the same level as the "Bass Booster" setting on the iPod) and the X5 is somewhere in the middle. The X5 has the most effective built-in FX among the three if you don't care about sound quality but just want lots of bass. The Monolith is somewhere inbetween all of this. If on the other hand you need to effectively but simply shape your sound to compensate for crappy ear/headphones, the Sony is the best bet.
If you use EQ all the time on your portable, it means a couple of things:
~SQ is not your priority, whatever you say.
~Your phone has certain deficiencies.
Now, based on which camp you honestly fall into, you've got to then start looking at compromises.
The X5's EQ only has boost. This means you don't have full control over the sound, regardless of the fact that it offers custom presets. However it does have other sound-altering options to make it 'sparklier' and certainly bassier. These have really quite major negative impact on sound quality, but to people listening on low-grade earphones the sound is brightened up a lot and made more dynamic. The Zen Vision M and the Sony both offer boost and cut on the EQ. The Sony has only three 'clicks' for boost and cut, but the stages offered are quite effective. The Zen Vision M has 1db clicks for boost and cut.
From a purely sound point of view, I would say if you have EQ switched on all the time in your set-up primarily to compensate for a crappy response out of a substandard pair of phones, the Sony is worth a look. If you very rarely need to use EQ and use it for subtle corrections which you want to control yourself, then take a look at the Zen Vision M. If you want to go nuts with sound effects, go with the X5.
None of the above players offer sound quality higher than the 5G iPod. However all bar the Sony (and I'll have to say that difference is fairly slight) will deliver comparable SQ if you decide to turn all the crap off