Even though I am a Playstation gamer only, I will admit that all of the systems are great and offer something different for different gamers.
In my opinion the worst system is the Xbox 360. Yes the 360 has great online play and has plenty of games to choose from, but the Xbox 360 is a flawed game console that has too many hardware problems. Microsoft still hasn't fixed the problem 100%, they have only fixed the problem a little bit by putting in different hardware that will reduce the chances of Red Ring of Death.
The console with the most to offer is the PS3 without a doubt. The PS3 has a good exclusive game selection now and in the future as well. There's good exclusive games for the PS3 that are for every age group such as Rachet and Clank, Heavenly Sword, Uncharted Drakes Fortune, Resistance Fall of Man, and Motorstorm. The PS3 is a console for casual gamers and hardcore gamers because of the diversity in exclusive games.
Most of the Xbox 360's good exclusive games are rated "M" like Halo 3, Mass Effect, Bioshock, and Gears of War. I think that the Xbox 360 is a system for mostly hardcore gamers and online games.
The Wii is a good system for everyone from young to old people and to experienced game players. But the majority of the Wii's games are for casual gamers and the Wii doesn't really have that many block buster game titles such as Grand Theft Auto 4.
In my opinion, determining which system is the best or worse should not be based upon console sales, but what console has the most to offer, and the PS3 has the most to offer.