Well i think it would be hard to put a stamp on 1 alliance, it depends on what selection i guess.
OE - I am sure i am not the most friendly player to them, but they certainly are a contender for this title, only a shade of what it once was. Mostly due to the next mentioned

TL - Not as old as some on this list, but certainly the alliance that changed the most over the years, still going strong although i don't believe the same concept applies anymore. The old TL and the current TL both great alliances, but not the same.
TA - Great long standing alliance, always stood up for themselves, but at the time OE was dominating i don't feel TA was worth mentioning. They tried but did not succeed to fight for a top spot in those days. Currently a super power though, and capable of doing great things.
DD - Although it had some great individuals, for me personally the only reason to be part of the GA because of their ascended status. Besides the few good people, also alot of statbuilders in their time. Were great on ascended, but not good enough for this title i think
DDE - Well similair to OE i suppose, they ruled the game for a long time. And still have great players in their ranks. I feel they are not the shade OE currently is, so this might be a good one.
Mayhem - Although a superpower now, i don't think they were infront even though existing since the stone age. Had great individuals in the old days, have some now, but never really pressed their mark on the history of SGW, so this one is gonna be out aswell.
Now you have others like RW, HVE and countless others among the years. I don't feel like they brought enough
yet to participate.
In the end i think i would have gone for OE, simply because they are the most long standing alliance out there. But because of the shadow it has become of its former might i will have to go with DDE. All this purely based on my own experiences and its just an opinion.
Can't believe i actually went for DDE though :p