I agree with Zidart in that I also think very poorly of the big three. Picking out the least stinkiest poo still leaves you with poo. Sorry for the graphic.
If I did, though, I would have to side with Sony (I actually did in RL too, after 3 years of sitting on the sidelines of this generation I eventually got a PS3). Free online, the best exclusives, free wireless connectivity (for the first couple years of the 360 you had to buy this, and it wasn't cheap...), rechargeable batteries in the controllers, 7 player local connectivity (yes I have used this - sports games), the fact that it actually worked and did not break down after 2 years, and non-proprietary hard-drives/headsets pretty much sold it for me. Blu-ray was nice, too, but not a big factor. I also think it will out-live the other consoles quite easily, which is good, cuz I am not buying another console again for a long time.
It wasn't as commercially successful as I thought it should have been. I still think 360 numbers are inflated, though, so maybe in reality it did comparatively as good as I thought it would. I definitely think it was the best of the big three. Wii dropped the ball big time with pretty much everything about their console, and M$oft should their true colors with the 360 with their monetization and advertising schemes (underhanded pricks).
Sony still screwed me (not just by having lax security), but less so.