I don't know if there's going to be a clear-cut 'winner' in this generation. By the time one finally begins to emerge it'll be onto the next generation.
The PS3 has taken a lot of negative publicity, but I think it could be a case of a lot of overreacting too early in the product's lifecycle. I think the situation of the PS3 and Vista mirror each other a lot actually. Both have massive existing user-bases from their previous generation product. Both of their previous generation products are very solid, mature platforms that still serve most of their users needs very well. Both have taken a lot of criticism in the media because people aren't migrating in massive numbers over to the new platform yet and that's somehow seen as a mark that the new product is inferior. I think it's mostly that it's simply hard to build that kind of momentum overnight with such a massive user-base who are already very satisfied with the existing product. But over time more people will begin to shift and it'll eventually reach a tipping point. I do think though that the PS3 is in a much more vulnerable position to lose a lot of PS2 customers to the competition than Vista is to losing XP customers to the competition.
The Wii is great (I own one and love it) but I don't see it as really a direct competitor to the other two. It's price point, technology and capabilities are just on a different plane than the other two. It's a fantastic value though and great for the casual gamer or somebody who's never owned a console system before. I can't help but wonder/worry if a company like Apple (or maybe even Google) might make a bid to snatch Nintendo up some time in the next year or so though.
The improvements and success of the Xbox360 are really impressive to me (and also make me eager to see what the next generation Zune might offer.) The tighter integration with PC gamers, the Media Center extender capabilities and the online network as a whole are very impressive. Assuming Microsoft continues to stay ahead of the other two in terms of product lifecycle then they have a chance to make a MAJOR impact at the next generation to become the first ultimate convergence box if they combine Media Center-like DVR functionality with traditional gaming and tighter integration with the Zune and/or whatever portable gaming device Microsoft launches. The fact that Microsoft is getting much more tightly-involved with the cable companies would seem to indicate this is where they are headed.