After a few years of buying used games, my PS2 collection is now my largest, followed by the DS. I've got quite a number of PC games as well.
However, when it comes to owning a large percentage of a console's library, I'm looking at about 80% of the Vectrex library now (including the 3D viewer and its games). I've yet to snag a Light Pen and its games and a few other odds and ends.