Is the liger the biggest cat in the world?
According to Sierra Safari Zoo (near Reno, Nevada), ligers (sterile offspring of a male lion and female tiger) are the largest cat. In terms of naturally occuring species, Siberian tigers are the largest.
Further search shows that male ligers are sterile, but not all female ligers are. This site has a table of possible cross-breeds with lions, tigers, and mixes.