All depends on how long you been playing EQ to say which guild is the biggest and well known to be honest. Each server is it's own little world, each one being a different world from the other and have different rules applied. I would even go as far as saying this also depends on the expansion.
For instance I could say Realm of Insanity and I'm pretty sure 98% of the people here don't even know who they are. They happened to be first guild to beat GoD, and also think they were only ones for awhile that beat it. This info is coming from SOE offical leader boards.
So in long run there is no way to compare cause ever guild has it's time of passing and another guild steps up to take it's place. This also holds true for some of the most famous groups ie... mana burn, shaman, necro, and rogue teams.