For me, there’s two. The first, is for the story and overall cutscene, and it’s EASILY the Talador finale.
From Maraad’s sacrifice to Blackhand being so full of himself he didn’t think Yrel and Durotan would work together, to Khadgar blowing the ship up. It’s just pretty great, haha.
As for the 2nd one, it’s a very polarizing cutscene, due to the content of it, but honestly, the voice acting in this cutscene, is by far the best we’ve had in WoW.
How about you guys?
It’s either “By our hand” or this one.
It’s hard not to choose a Sylvanas one, because I just love her voice acting so much.
This has always been a favorite of mine. Oh, you said in game? Whoops.
In game is easily this one. The emotion in Jaina’s arc and redemption is one of the best arcs this game has ever done. Kul’Tiras is also an incredible island and story. The Daughter of the Sea.
That’s funny because those two are my exact favourite cutscenes too.
Cinematic-wise, WoD will always be the best for me. The story potential was amazing but they squandered it. 
I vote for this one.
Broke my little heart to first time I went thru there and I still cry each time I see it.
Wrathgate.
Garrosh’s final scene. It was powerful and it sums up how I feel about stuff.
Blizzard: YOU WILL SUBMIT.
What blows my mind is despite the horrendous story telling Blizzard has done recently, with Sylvanas and all the other mishaps.
This cinematic right here was a perfect send off to Garrosh and it completely did him justice. What an awesome moment.
Ysera’s death. 
Warcraft’s cheesy writing isn’t supposed to make me cry!
Shocked the 
out of me when it happened. When it was over, I scrambled for Google because I thought I had to have misunderstood.
(I hadn’t.)
Ysera’s death and the rebirth in the Ardenweald one.
They’re both just so tragically pretty. And I stg I thought the winter queen was gonna sacrifice Ysera to feed other groves. Her voice actor really nails the character when it counts.
Don’t have one, when a cutscene pops up I instantly skip
I have shed many tears while watching Blizzard cinematics. There are so many of them that just touch my soul. There is one in particular though that is more powerful than any other I’ve ever seen in the game, and that is the scene where Xe’ra attempts to impose her will upon Illidan.
That cinematic received more ridicule and scorn than any other I can recall. People mocked it, laughed at it… endlessly. But not me. It is an extraordinary piece of cinema, thought provoking and deeply profound.
Seconded. While the visual quality is dated seeing Putress roll out with the goo gave me tinglies.
“Did you think we would forget?!”
Watched it 50+ times always gives me goosebumps.
I will always be a fan of the wrathgate.
All of the scenes where characters sacrificed themselves.
The Siege of Orgrimmar patch trailer is a personal favourite of mine.
As for cinematics that can be watched in game, that would probably be the Anduin cinematic from Legion
Or Warbringers: Azshara (which you can watch in game by clicking on the following NPC https://wowpedia.fandom.com/wiki/Memory_of_the_Pact)
I do love how the music used in that cinematic is the same music that played when Garrosh resurrected Y’shaarj in the patch 5.4 trailer.
Garrosh: I answer… TO NO ONE!!!
Two of my other favorites already posted, here’s my #1 pick. This is a remaster, obviously, but the orignal is still so good.