Still Alive by FAR. Want You Gone just seems like a desperate attempt to match Still Alive's greatness. The lyrics aren't really funny or witty or anything, and the music brings with it this sense of over-exertion, like Coulton tried way too hard.
It's not even catchy! Still Alive was FULL of awesome little references and GlaDOS's personality absolutely shone through the lyrics, on top of the awesome little beat it had going. Want You Gone instead has a more conventional and slow paced lyrical style which doesn't really have anything interesting going for it. Again, it just feels like he tried to match Still Alive but didn't end up getting there at all.