ur options:
classic era - base vanilla version of the game, servers released in 2019, slow pace leveling, no changes, old economy
sod era - vanilla but with some changes, currently on phase 3/4 lvl cap 50 (will be 60) people seem to play it for like 2-3 weeks when phase releases and then population drops, so wouldnt really recommend until phase4 releases, also classes were changed a bit compared to normal era, spells added etc
both sod and classic are separated from classic expansion such as:
cataclysm classic - just released after previous classic expansion wrath of the lich king, u level way faster than in vanilla, also its easier, 90% of the expansion content is still unreleased so now we are in "pre raid gearing phase" , idk would recommend it, because its not as complicated as retail, and not as stale as classic era, also lot of people cuz its fresh
retail - idk i dont play retail, its newest non classic expansion, leveling is pretty umch a speedrun here, u get into the endgame and have to figure shit out i guess, might be difficult if u dont have friends cuz its bit more complex compared to classic
also there will be new retail expansion in like 2 or 3 months iirc so maybe wanna wait for that and get some friends to play 2gether
also to playy any of the classic modes u jsut need to sub 13 eur/month
if u wanna play retail u gotta buy the dlc + sub
tldr i recommend cata
playing it from fresh cuz i havent played Wrath and having a blast so far
tldr : cataclysm classic