You can't rate SL against finished expansions. But purely from a X.0 patch standpoint SL is the best expansion I have played (did not play TBC or WotLK).
Cata's first patch was okay, Raids were kinda meh but heroics were fun and challenging. Also it was the first time I really played WoW. MoP's first patch was dreadful, boring raids and daily quest hell. WoD's was great for one week and then boring. Legion's was absolute trash. Having to play with half assed classes sucked big time, especially because many classes got pruned into oblivion. BfA's was even worse, Classes lost everything from Legion and did not gain anything new. Also Azerite scaling bullshit.
SL is just perfect in comparison.