How the hell is MoP leading? MoP has it flaws like how Garrosh suddenly becomes totally crazy after seemingly going in the other direction in Cata (I could of easily accepted WOTLK Garrosh becoming MoP Garrosh but not Cata Garrosh) but the story is decently told and other than that leap makes sense and is consistent. It also has the Mantid, Lei-Shen who is probably one of the best done side villains, and Pandaria itself has tons of other great little bits of lore. People are just blinded by "PANDA'S WAAAHHH" (by the way though they aren't as well done as some other stuff the Pandarian's get some solid bits of lore too, and a more established culture and society than a good bit of other older races).
The correct answer is TBC, almost nothing made sense in TBC, And it's derailment of characters was by far the greatest and the worst, especially Kael'thas where absolutely nothing resembling his previous characterization remains, I still barely understand why we were even fighting Illidan beyond "the Shatari told us to" He may of been made into more of a dick than he was in WC 3 but he still wasn't a threat to Azeroth at all, he was building an army to fight THE BURNING LEGION not us. It made almost no sense to fight him when we have at different points allied with people significantly more evil than him to fight a greater evil (like we did with the Mantid). And then we had Zul'aman a cool middle raid to be sure, but one with pretty much no build up at all. Firelands, Ulduar and Throne of Thunder were being built up to in the earlier patches of their respective expansions, Zul'Aman had maybe a couple of quest in Ghostlands as lead up.
Pretty much the only good lore in TBC is the Blood Elf story apart from Kael'thas, the Akkora and the Mag'har stuff. The Dreanei are cool too have potential but they've never done enough with them
Cata started strong but became a mess toward the end, but it never reached the levels of nonsense that TBC did. And at least Deathwing's character change could be explained by him being totally subservent to the Old Gods now and as such much crazier than before and less of a schemer because he was now carrying out someone else's schemes.