Soundwave on 31 May 2009
PS2 simply because it had all the third party from day 1.
With Wii developers are still adjusting/trying to get better content for the system 2 1/2 years in because not many developers (hell lets be honest -- no one, probably not even Nintendo) could've forseen the system being this successful.
The Wii was coming off the GameCube remember.
That said I think the Wii will have an equal to or better than library to the DS when it's all said and done. The third party support is getting higher in quality this year especailly and with things like Punch-Out!, Monster Hunter Tri, Wii Sports Resort, Grand Slam Tennis, Little King Story, Final Fantasy: Crystal Bearers, and a new Mario coming I think the library is starting to fill out nicely.
