Depends on what you're looking for in regards to raiding.
In terms of end game content:
Riot has 24/7 coverage, can and does kill everything
Azure Guard has next largest window of coverage, can solo all content as well.
Freedom can do a fair load of the content solo, has smaller numbers so smaller active window on larger targets.
Kittens Who Say Meow can do a fair load of content but only go for choice endgame content when most opportune.
The Second Sons can do a good amount of content, generally goes for end game velious on quakes only.
Aegis/Dawn Believers/Lighthouse/Castle all can do content but smaller windows of contest.
All of these guilds have regular-ish HoT farms, Plane of Sky rotation slots, and are apart of an ongoing Plane of Fear golem competitive rotation.
Really depends on what you are seeking out. Most every guild would take you at 55, though I am not certain as I know Riot had more strict requirements for applicants as of recently.
Azure Guard is always recruiting, if you wanna check us out
head over to our site and maybe drop an application!