Hephasto and Nihlathak are the best zod droppers in the game (except for Baal and Diablo with their first drop), but anyway: You have to kill Hephasto for an average of about 3 million times to get a single Zod rune while 3,000,000 forge rewards will give you an average of 272727 of each rune from Hel to Gul which can be cubed up into about 1000 Zod runes. Rushing a HF dummy obviously needs less time than 1000 Hephasto runs, so going for forge rewards seems to be the more rewarding thing, even when naively assuming that it's worth to cube up runes from Hel-Gul up to Zod. :azn:
On non-ladder the case is different, of course, but it would still be far more rewarding to kill random monsters in high-level areas than hunt down a single boss. Hephasto's chance to drop Zod is about 6-7 times better than for the ghostly monsters like Gloams, Souls, Storm Casters etc. in high-level areas, so killing e.g. 10 Storm Casters around Hephasto (or simply anything in L85 areas like the River of Flame) will have a better chance to produce a Zod rune.