Guardians have been the core of the meta since the game was released. No other class has that much AoE support or at least none that compared to a Guardians. Firebrands just do what they always did better. Necro's pretty much the same, always been in the meta because nothing else can corrupt on the same level, Scourge just adds to what a Necro could already do just more effectively. Rev came along and pushed Warrior out of place quite a bit, Spellbreaker brought it back in. They also pushed Eles out even further because they have better sustain and on par if not better DPS. Scrapper became a thing because ANet over-buffed it with the gyro change, before that patch, nobody played Scrapper. (Or very few to my knowledge, please don't hate me). Mesmers were used for Veil pretty much, not much else until Chronomancer became a thing then they became Boonbots pretty much. Thieves and Rangers have never really had a part in the 'blob meta' due to how their classes play/designed. However, Thieves have been a part of the less popular GvG meta since focus parties become a thing (Not Commander sniping, wrong idea!) and now they kind of have a place due to Daredevil's ability to bounce around evading with Staff 5.