Sony. People seem to forget them fairly easily, but they have everything they need to beat Apple at this game and soundly(they actually manufacture almost everything needed to make their own tablet in house, they own an enormous music studio and several movie studios).
Sony isn't really in that good a position.
Prior to the Ericsson acquisition, which... happened just now, they lacked the capability to design their own phone devices, and needless to say, all the components that went into those phones except for the screen, and some other specific stuffs. But I'm sure as heck they didn't have the capability to write their own mobile software, or design their phones' hardwares.
If that's any indication, their tablets likely follow the same process, so they are already one step behind Apple in the design process.
As for being a media platform, Sony lacks that as well. The only thing that they have that can be remotely considered a media platform, is PSN, but it's mostly for their gaming consoles, and they can't just bring it over to their tablets and phones overnight. Apple has had iTunes on their devices since day one.
I don't think anyone can dismiss the stretch of Sony's prowess as an electronic giant, but I dare say, their mobile products have been lacking as of late, and to say they have a fighting chance is like saying you believe a fat rich kid with no training can defeat a dragon.
*IF* the tablet wars turned based on media consumption(something I honestly find highly unlikely) then I would say long term Sony is rather easily the best positioned. Media delivery/consumption are certainly going to be important factors, but I see that as one of many.
The tablet war has already turned media consumption.
What else can you do on a tablet but consume media? And that's aside from a small niche that consists of professionals making music and fixing photos, which, if I might add, is already exclusively a signature of the iPad as other tablets lack the same app support.