Well it depends on who you ask. Some would say apoc, because a large number of their players are hardened fighters and not afraid to get dirty. Others would say UC for their numbers, and demonstration of power in the OSW's of the past. Others still would argue WDGAF is the greatest, due to size, as well as it's history in some of the bigger OSW's of kaw.
Umbrella corporation. A massive amount of clans involved, I believe it was around 23 last I checked, though things change. Tough clans like resilience and clan A make them a powerful group. Lot of good warriors over there.
Zaft is in WDGAF. Zaft stands for Zodiac alliance freedom treaty. The Moto i believe is WFTT (whack first then talk) WDGAF stands for We don't give a **** if I'm correct.
Zaft is allied, not in wdgaf. And I'm 99% sure UC is no longer a thing. And the guy that said their an old alliance...they were the newest one that broke up after their first osw.
Any clan who will fight another, and/or defend it's members are all worthy of being called the greatest. Only things I never liked about OSW clans are; clan/alliance politics (regulations, requirements, etc.) and trash talking (only makes you look bad). I, myself am not an OSW player in the slightest, but I have the utmost respect for them.