Wii U, easily. If we're talking very recently (like within the last year) then I suppose the Wii and PS2 are worse, but I still have plenty of backlogged games for those two systems and I've played hefty games on both already (mainly half of Xenoblade for Wii, but a whole bunch of Final Fantasy games and Persona 4 of PS2). Meanwhile, the only game I've bothered to finish on Wii U is Lego City Undercover, and that was in 2014. Which means I barely touched the thing from launch until this year.
Oh, wait, forgot about the 360. I did play GTA V on it last year, but that and Child of Eden were the only two games I played on it in the past year and a half at least.