Chris Brown goes with ease. It's not that he's wack. I think he's made some dope songs. He's the odd ball out because he doesn't have that one definitive song or album. With Usher, you have Confessions and songs from 8701, Confessions, or even albums like Looking For Myself and Raymond vs. Raymond that qualify. Bobby Brown has Don't Be Cruel and "My Perogative". Ginuwine has The Bachelor and "Pony". Justin Timberlake has Futuresex/Lovesounds and "Cry Me A River". These are songs and albums that have been influential on other artists and had huge impact when they came out. They were beyond just hit songs, but time tested.
The other thing is that besides dancing, there's not one thing that he's better at. Bobby's a better singer. He also at the very least co-writes and co-produces his own music. Usher's a better singer than Chris. His catalog is also better. Ginuwine's a better singer. He's also written and produced his own music. I give the edge to Chris over JT in the singing department. Justin writes, plays instruments, and produces giving him the edge overall.