Zune is great, but won't work with your Mac. iPod's sound quality isn't really great. I'd say, "Average" or right below "Above average" at best until you iMod/DIYMod it (which wil get even the most discerning audiophiles to say you basically reached the peak of DAP sound quality).
The Archos 5 is more of a video player, but still a bit better than the iPod (touch) in sound quality. But you wouldn't want to use headphones that cost as much as the player.
Nobody really does high capacity anymore. It's horrid. DAPs have went backwards. Good luck finding a high capacity that's going for sound quality. They all now try to use flash memory and put in too many bells and whistles.
You can try to get the iRiver H-140, but good luck with that. Sure it has 40GB (which like the iPod, you can mod it to get 240GB), but it has features to being a true audio player compared to most like the FM tuner and
digital out. But being discontinued, you're way back in line with audiophiles doing everything they can to get it.
Head over to head-fi.org and go to the "Portable Source" section. Sorry about your wallet.
Portable Source Gear - Head-Fi: Covering Headphones, Earphones and Portable Audio