The first thing you should realise is that you should take
anyone's opinion on this sort of stuff (even mine!) with a pinch of salt, not just because these things are so subjective, but also because making a meaningful comparison between models is much more difficult than people realise.
For example, you tend to perceive sound with more top and bottom end as being higher quality initially, although with extended listening this can be fatiguing - which is why cheap hifis are always so sparkly and subby, to fool you. So a quick a/b test may not be accurate. Also, very tiny increases in level can have a pronounced effect on high and low end perception (you perceive louder things as brighter and fuller, but the mids don't feel like they increase at the same rate). So
very slight, almost imperceptible differences between the volume of two devices might make you think one sounds better than the other. Also, a high-ish noise floor (i.e. a tiny bit of hiss) might initially make you think one device sounds worse. But your brain tends to filter out this sort of constant background noise over time, so it might skew your opinion against a better-sounding product.
And those are just three quick things off the top of my head. But having said all that, and on the understanding that it's totally unscientific and just my impression without serious testing, I'll give you my opinion anyway
I am a professional audio engineer (see "my job" in the sig), so I have well-trained pair of ears, but when I listen at home for pleasure I am not an "audiophile" (I think I just threw up a little bit). So in terms "listening for pleasure" without getting too picky about it, I think they all sound pretty good (My main experience is with a 2G, a 4G and a 1st gen iPod touch).
But if you really pushed me, I'd say that the 4G is not quite as full sounding as the 2G (but maybe it doesn't go as loud? And maybe I used to keep more songs as AIFFs? And maybe I changed my headphones at some point? I can't remember). I am pretty sure, though, that the iPod touch sounds better than the 4G. Slightly punchier sound, better transient response - and that's definitely with the same files and the same headphones. If I had to guess, I'd say they use better quality components on the earlier models of each form factor, as that's when it's getting reviewed.
But honestly, they're all pretty good, and a good pair of headphones makes 20x the difference between the best and worst model of iPod (and if you're connecting to a hifi, do it with the dock connector).
