IMO it was the PS3 in the end. Not being able to play games like The Last of Us, Uncharted 2, Demons Souls, Little Big Planet 1&2, Heavy Rain, Metal Gear Solid 4, Valkyria Chronicles, Ni No Kuni, inFamous series and so on - thats just WAY too much to miss!
Nintendo Wii is close second with very few VERY amazing games. Super Mario Galaxy 1&2, Zelda SS, Metriod Prime Corruption and maybe one or two others. Very few games, but so amazing that you just can't miss them if you want to call yourself a gamer.
PC is third thanks mainly to competetive gaming. Dota 2 and Starcraft 2 are simply amazing games, although they are not for everybody. It's worth to play them at least a few times just in order to understand whats going on so that you can enjoy watching the esport tournaments.
X-Box 360 has its fair share of great exclusives, in particular Halo and Gears series, but overall it ends up being last. It has a lot to do with the fact that most 360 games are also playable on PC though.