So that's the "best episode ever." Huh. Well, from someone who's watched every episode, I can say that "Asian F" isn't the best episode, not even close. It's not bad or anything, just nothing special. The structure of the episode is about what you'd find in a normal Glee episode--a bunch of plots, some hitting the mark and others missing by a wide margin. I did think, though, that the highs in the episode were higher than usual and lows weren't as low as usual (a big part of that can be attributed to Sue's absence).
Harry Shum, Jr has been waiting in the wings for two seasons and he finally got his chance to shine. His scenes are easily the best part of the episode, shedding light on a character who's basically been the generic dancer. We get to see his father, who wants him to focus on school, and his mother, who gave up her dreams of dancing in the past and now encourages Mike to follow his dreams. It's a little schmaltzy, but there was a lot to fit in the episode and learning about Mike is better than nothing.
Glee's randomness is evident in the Mercedes plot. It's kind of random. Prior to the episode, there wasn't any indication that Mercedes was this unhappy about her situation or any build-up to the explosive moments in the episode. So when Mercedes does quit New Directions, it feels awkward. Yes, she probably should have gotten the part for Maria, but Rachel has taken plenty of parts from her in the past. What makes this instance more extreme? Her reasons for going uber-diva are hazy at best, and at worst motivated by her boyfriend who randomly pops up.
Will and Emma are the worst part of the episode (no surprises here) and it shows how set the writers are in their ways. There's some crazy shit about Ginger preservation/supremacy which causes Emma to go rub her hands together, leading to "Fix You" in the hilarious, "emotional" music video format. It was all so silly and ridiculous that I just chuckled at how stupid it was.
While I don't think "Asian F" is the best episode of the series ever, I can see why Fox wanted to promote it that way. It has these big moments where things are very emotional and I can imagine some people fainting over them. But when those moments are examined as part of the bigger picture, they make little sense and serve as a reminder that plot developments are all contingent on the writers' changing whims.