Ack... more fighting words... I own all three systems and have since day one. I own 2 Xbox 360's. Here's the truth as I see it on these systems.
1. PS3 -
Pro's: Blu-Ray movies, Awesome DVD Upconversion, Solid Video processing all around for supported Codecs. Solid 5.1 audio, relatively quiet. Has a few worthwhile exclusives. Built in WI-FI (although I still prefer wired)
Con's: Blu-Ray increases loading time in games. Multi-platform games tend to play & look better on the Xbox. Online multiplayer is poorly implemented. Six Axis controller is crap. Blu-Tooth implementation is stupid, and the lack of IR means you have to buy an IR adapter to use with Universal Remotes. Only supports Fat32 file formats for external devices meaning you can't playback large video files and can't connect your ipod. The user interface is pretty awful. ALL of the add-on apps are poorly done (like the internet browser)
PS3 Wins = HD playback and Upconversion of other Sources.
Xbox 360=
Pro's: Games load faster, more A+ exclusive titles, much better user interface, Multi-platform games play and look better (in general), better controller (in fact, the best in my opinion) Excellent HD-DVD playback (yeah I have that too). Decent playback of supported video files. Support for HFS+ format so you can playback HD quality MP4 movies and connect your iPod (I don't have an iPod) Standard IR receiver that works exceedingly well with universal remotes. Xbox Live, yes it cost's $5 a month, or $3 if you buy a card from amazon. But the service blows away the competition. Netflix streaming video. Awesome integration with Windows Media Center.
Con's:
Reliability? I hear a lot of people have problems but I have never had any problems with either of mine, nor do I know anyone personally that has had a problem. First gen model is noisy. Newest gen is actually quieter than my PS3(gen1). No way to hook up HDMI and Toslink cables without taking the plastic off the adapter casing. Does not upconvert non-HD sources nearly as well as the PS3. Proprietary memory cards. No built in wireless.
Wins: = Streaming Video, Gaming.
Wii:
Pro's: It's small, it's quiet, motion control has some potential. Comes bundled with a good game. Built in Wi-Fi. Uses SD cards for memory. It has a few good games. Nice backlog of downloadable legacy games.
Con's: Hardware is a generation behind, graphics are terrible, most games are worthless, media playback is nonexistent and/or terrible.
Best resolution is 480P. Doesn't come with component cable. Wired sensor that has to sit on top/bottom of TV. Lack of wired Ethernet connection.
Wins: Marketing. Introducing new people to gaming. Making gaming less dorky to outsiders.
PC:
Pro's: PC is still pretty much the God of Gaming.
Con's: Cost's at least twice as much as any other platform. Some console ports have been pretty weak lately (GTA IV).
Wins: Multitasking, Customization, Graphics, Sound.