Alot of bad and incorrect info in this thread!
The zotac is the best if you want a slight overclock and save a lil money plus not really wanting to overclock very far, lightning and classified are custom PCB with loads more VRM(better voltage), more stability with overclocks and actually way better clockspeed (outperforms titan for half the cost?), in a nutshell the classified and lighting are built like tanks and made to overclock(dual bios one bumps up the thermal and power threshold, 1 is a stock higher overclock), so if you screw up, your covered. Clocks of 1.3ghz are common on the classified(on air!), zotac won't give you that or replace it if you screw it up trying to hit those speeds. The zotac is competing with the superclocked 780 from evga and the twin from MSI, not even the same tier.
TL😀R Classified and lighting are made to overclock to your hearts content, zotac is just a slightly overclocked reference 780 with a better cooler.