I don't know whether or not LR could be considered a spin-off of the XIII series. On one hand, like most FF spin-offs it only focuses on one character instead of a group. On the other hand it's the main character of the series.
Anyways, right now Dirge of Cerberus, but I'm hopeful that LR will change that.
Sabretooth: "End of the line, runt. Got any last words?" Wolverine: "Yeah, two." SNIKT!!! SNIKT!!!
Dirge of Cerberus isn't a spin-off. Kingdom Hearts isn't a spin-off. Advent Children isn't a spin-off. Lightning returns isn't a spin-off (WTF?). Crisis Core isn't a Spin-off. Type-0 isn't a Spin-off.
Basically all Sequels and Prequels are not spin-offs.
Dissidia, Tactics, All the Bravest are among some of the spin-off.
LR doesn't count because it is a sequel. Kingdom Hearts doesn't count because it's another game not directly related to FF. Type-0 doesn't count because it is a mainline title. The 7 sequels and prequel doesn't count because I already stated the reason.
Ok Dissidia 012 is my favorite spin-off.
Versus XIII is coming in 2014, GTA5 will be awesome, and PS4/Xbox 720 is coming in Dec. Lightning Return will be super awesome and I don't care what you think!