What should I buy to begin playing complete edition, BFA, or shadowlands??

there are retail version and classic

Retail (BFA/Shadowland) is fast-paced, less grindy, and easy to level. if you do not have stable time to commit raiding hours, this is a good version. because u can go raid using the dungeon tool.

Classic is like old times, from start til end u need to plan carefully. and u migh need to commit raiding hours. for example saturday night 8pm means 8pm, because guildies will be waiting for u. unless the guild has more than enough they can proceed.