I used to play with a guild that had SoO Cutting Edge achievement in MOP ( im 31 this year feels sad ) - first as Arcane Mage in the first 3 raids switching to DK frost dps in Throne of Thunder and Monk tank as 2 mains onward ( i switched only because i loved arcane and fire started becoming better) . Since MOP was still very popular back in 2012/13/14 we had 2 raiding rosters active and 6 raiding days a week for those who wanted to play alts the main raid was 2 to 3 raiding days depends
Guys dont play affliction warlock from the beginning simply because its OP on warcraft logs in SoO .Demonology warlock was top tier at the start of the expansion as i can remember . But the most reliable in terms of dps in the beginning of the expansion im talking up to Throne of Thunder raid was Arcane Mage . Arcane Mage in the first 3 MOP raids was bullying other dps out of the dps window really . Other dps were doing like 20 % less dps then Arcane Mage on avarage and later Fire Mage is pumping still . But dont let it stop you from playing a class that u wanna play.
In terms of playing a healer as far as i can remember priests paladins shamans were top tier unfortunately druids were not that good they always lacked but people still played them and u still can get into a raid with them. Disci priest and Paladins are the best choice if you wanna get invited asap into the group. Also these 2 were “meta” if i can recall while raiding TOT and SoO
In terms of playing a tank . Paladin tank was CLEARLY the most OP tank in the last raid of MOP and i still believe it. Other tanks are also very good and you can play everything you want . The only classes that had a little disadventage were blood dk with their HP SPIKING A LOT during heroic raids warrior tank being VERY GEAR dependent . And monks had problems with magic damage taken especially DOT’s since their stagger did not affect that . I hope they will make proper changes and adress these problems. But in terms of physical damage mitigation monks are WOW . I dont remember exactly how monk was played it was 13 years ago but its like riding a bike you will never forget it .