In terms of the versatility, quality, and stability of video playback, my un-nuanced ranking of the following devices:

1) Archos 605Wifi
2) Zune (2nd Gen, 80gb)
3) iPod Touch
4) Sony Mylo Com-2
5) Nokia N810

I own all of these devices (I'm a wifi-gadget obsessive). This is a rough estimation with subjective reasoning rooted entirely in unmeasured anecdotal observation. In my very humble opinion, the Archos destroys all of the competition.