I've got reasons to like each system.
Wii - It's got a few exclusive titles worth playing, Netflix is implemented better than on the 360, it has the types of games my girlfriend likes, so since I'm more flexible about the games I'll play than she is, it's something I get to do with her. Bonus that it's her Wii and I just had to keep my GC controllers.
360 - It's got Plants vs Zombies with Co-op and vs Mode. Other than that, really just a few other titles that aren't available elsewhere, or run poorly on the PS3. I prefer Catherine box art for the 360.
PS3 - Online multiplayer is free, no stupid points system for buying content off PSN, blu-ray and home theatre system integrated, easy ripping of music CDs, massive list of exclusives I like. More of my friends like the PS3 than 360, Wii comes in second, 360 third.
PC - First Person Shooters, I love them. At this point I haven't had a chance to try Move shooters on PS3, but I really want to see how it goes. Free online multiplayer is also great, along with the rest of the strong points of the PS3. I'm not so fond of my laptop overheating in FEAR though, but that's more that it's a crappy laptop with the same quality as the average 360.
OnLive - It doesn't make my laptop overheat, so I can keep playing First Person Shooters. If I had a PS3 right now (I just used my roommate's before), I might not care much as I'm mostly playing UTIII on OnLive.
PS3 & PC are my personal preferred systems, they both have a strong overlap in features (IE, tons of stuff the Wii and 360 just don't have), and they've got the largest list of exclusives. Between the two of them, a 360 is pretty pointless to own. The Wii is kind of like the PS2 - massive catalog of games with backwards compatibility. The PS2 has an excellent selection of RPGs, the Wii doesn't have an excellent selection of anything, but it does have enough titles to make a good number of random games that are fun to play.