It's easily THE ARMY (yes, I edited the name of Britney stans because we don't use "britbots.")
If you know anything about ATRL, you would know The Army was one, if not THE driving force behind the site. When Britney stans started coordinating, "uniforms" aka matching avatars for major Britney events, others were "inspired" with the release of their favs. When The Army created their own Twitter stan site and tumblr feeds, others were "inspired" as well. As a collective group, The Army holds the most influence, individually the Beyhive would hold that title.
Honestly, the two most influential stanbases on ATRL historically and presently are The Army and Beyhive. The Beyhive benefits from having several standout members like Sats, Geezus, Kisuke, Drais, and such that puts them up there. Those four alone hold big influence on the site.
The biggest thing I think the Monsters contribute to the site is just numbers.
