Its still way too early to say, since post launch support is what makes or breaks an expansion. Personally, I think MoP is actually the high watermark of WoW expansions but strictly comparing launch content I think Legion has a stronger offering.
MoP is my favorite expansion. As a casual you felt you could still play at your pace and still progress your characters. LFR as mind numbing as it was, it still had a semblance of fight mechanics (Garalon for example still wrecked pugs at first), something I imediatly noticed was even more dumbed down in the first WOD LFR, where you literally had no tactics. You could also get your weak ass tier but it was still tier, you still had cool bonuses and could still look cool, removing tier in wod LFR was a huge kick in the balls for casuals imo.
The raids themselves also felt goof "for realsies" as far as I could tell, I actually raided 10m with a healing monk in the first tier (mogu shan, bug place and terrace) for a bit and it seemed good. Expansion was also alt friendly as shit. Just overall a great expansion imo. I said it before, raider Rhak holds TBC in his heart as the best but current casual Rhak prefers MoP.
Legion seems to be hitting all the right notes for curreny Rhak, from returning tiwr to LFR to lots more single player content in Surumar and world quests, to lots more 5man content that I dont need a guild for, I just need the few friends I have left in the game. Hopefully its as good as it sounds.