I think we're stretching it with "best expac ever", considering MOP really pushes a lot of things that run counter to what people often say they want from an MMO. (And if you aren't playing casually, this xpac left you feeling alienated.)
I'm not sure I know which one is the worst; the lore isn't bad in MOP, but WOTLK I think was way better. The heroics/instances aren't terrible, but scenarios are fairly shitty content that you'll do once for achieves, so what you're left with is heroics that don't hold a candle to TBC. Admittedly, they changed the formula, and challenge modes are awesome, but I feel a little let down in this respect. It's all the same few instances, just you choosing to do it in a specific way.
They push daily quests and rep, both of which were fine in small doses and done correctly, though they did neither.
Pet battles are tons of fun, but it's pretty much a carbon copy of pokemon that they managed to copy with errors.
I also don't really understand what the big thing about Brawler's guild is? I'll do it until I can get clock'em, and then I'm out. Our server has a small group of people who are really into it, and when you visit you'll see them, but things like invitations on the BMAH aren't even being bid on.
Best or worst xpac is all opinion; the things that I've had the most fun with are the throw backs to EQ honestly. I've farmed all of the rares for glorious and I'm one item away from the Relic Hunter title. I don't touch many of the dailies, and the dailies are a large part of the content. Shieldwall dailies are utter shit - they're only good because I want my trinket/ring, and occasionally I get a real quest that gives me 440 rep instead of 165, and that's encouraging. Whoever designed the dailies inside the fucking shrine should be shot.
So really...it's all completely biased. I like parts of MOP, but personally, it's far from the best xpac, if for no other reason than everything you do clearly shows their goal for all of the activities they have you do:
Subscribe longer.
For the record, I liked TBC for hard instances/raid dungeons. I like WOTLK for questing and lore and for some relatively cool new functions. Cata had a bunch of cool class abilities, and the entire world changing was great. MOP has...asian themed lore and music, and pokemon.
Give me an xpac where they combine all that shit and I'll go for it.
I imagine if you're playing super casually like posters above claim it's great, since the xpac heavily relies on you doing the same things over and over again; for people who want to put a lot of time into it, they'll burn out quickly. But anyone who logs in, does a bit here and there, and has something to shoot for, you're mileage probably greatly varies.