I think Cata was definitely the most fun expansion just for doing regular bgs and stuff- the classes were more skillful than in either WOTLK or MoP. In WOTLK, there's a lot of times when you die to some dumb skill-less mechanic like a shadow priest spamming devouring plague or a ret paladin with a castsequence macro for their few damaging abilities. In MoP, they handed out all kinds of skill-less OP goodies like second wind, burst of speed, stampede, psyfiend, paralytic poison and my personal favorite, that dumb warlock succubus pet with a cc with no cooldown (how that one ever got approved I'll never know).

I think Cata was the sweet spot for pvp- people don't think of it very highly because its pve and lore was way behind WOTLK, but the pvp was really good imo.
I agree, I think cata overall was the heyday of pvp, despite competitive wow having died after s8 (mlg dropped it and then it kinda just faded from there, now all that's left is blizzard's championship series every year). I never pve'd in cata so I can't say much for that, although I agree that the lore had a ton of holes and WTF moments, but it was a story never before told in warcraft or any novels, so it wasn't as well articulated as something like the story of Illidan, the Sunwell, or the Lich King. MoP was basically cata's ugly younger brother, the PvE wasn't tremendous, the lore was pretty eh (excluding the side stories, Vol'Jin, Wrathion, Taran Zhu, etc. I thought those were pretty cool) and the pvp was god awful from the get-go and never recovered (see: Mitch Jones' Big Numbers videos or Reckful's video of Pam playing his warrior at 2.4k rating).

While I thought wrath was great, I really never was any good at it and going back to learn it now would take me weeks of time given the time I have available to play, so while I downloaded it and play it from time to time, I really can't enjoy it as much as cata simply due to the fact that I'm hopelessly terrible and get destroyed whenever I set foot in an arena, whereas in cata I played a ton and got pretty decent at pvp (iirc i was around 2100 mmr on bloodlust US, and 1900something soloq on AT) so I can do fun things like playing 2's with warglaives on, simply because my understanding of the game and mechanics/cooldowns and such is so good. I'd really hate to see that go, but meh. I'd probably just put wow behind me altogether and learn a new game.