While I don't agree that HMs aren't challenging, I would point out that there is a lot of skill required in navigating archaic controls and clunky combat design. Doing calculus by hand takes more skill than using a calculator, but it doesn't make it more efficient. Just makes it unnecessarily complicated and obsolete.
For me: Wrath > TBC > Vanilla > MoP >>>>>>>> Cata.
Wrath was the sweet spot. Emphasis on community, great dungeons, great raids, awesome zones, memorable music, great pvp, Wintergrasp, optional dailies, epic feel. They struck a close-to-perfect balance between farming gear and rng drops. Closest balance without overly homogenizing buffs and flavor. Closest balance between casual-friendly and hardcore content. Talent trees were the most fun/interesting. Stat scaling and stat variety were the most fun.
Cata ruined everything I loved about Wrath. Too much homogenization, fake "flavor," slow content, bad content, community-ruining features, guild-ruining lockout merge, annoying leveling. EDIT: Launch raids and dungeons were flawfree. Grim Batol, cloud dungeon, Halls of Origination were remarkable.
MoP did not feel epic. Ugly gear, ridiculous leveling content (killing rodents and picking veggies? Really?), eye-rolling premise (sha, alementals, etc), and dinosaurs everywhere. Music is godawful. Trolls are everywhere as if we haven't been fighting trolls every single expansion for reasons unknown. The whole thing feels like an Asian Journey to the Center of the Earth, and not in a good way.