I was thinking long and hard about this question, and I realized recently that the Royal Wedding Rehearsal carries a similar theme to a specific minigame in the game Silent Hill: Shattered Memories. In the game, your character has several therapy sessions with Doctor Kaufmann in order to help with your past; in one minigame (which starts at around the 7:40 mark of the video below), you listen to a story which involves four characters where one of which dies, and you have to arrange the four in order of how guilty they are of the action:

This reminded me a lot of the Royal Wedding Rehearsal, because in reality, all of them did wrong, including Twilight. Because of that, I wanted to list my order of who acted the worst, starting with the least:

1. Shining Armor
-I know, I know. He WAS the one who kicked her out of the wedding in the first place, and acted pretty nasty about it too. But let's remember one very important detail: Shining was brainwashed! Up to this point, we had absolutely no idea how much he was affected by Chrysalis's spells, or how long he's been under her thrall. And even with the idea that he wasn't under the spell during the rehearsal, all that he saw was his younger sister causing a ruckus, making his fiance cry, and nearly ruining his wedding. Wouldn't YOU be pissed off too?
Besides, for all we know, his outburst may have only been a temporary thing. He may have easily felt remorse for what he did and try to look for her, if he hadn't been under enough of Chrysalis's brain-washing to end up looking like a zombie by the time the ceremony began! Despite how he acted at the rehearsal, I think it's clear that he was clearly under influences beyond his control, and was clearly out of his element.

2. Twilight Sparkle
-Yes, she was technically right that something was wrong. And yes, she was correct that the person she was accusing was evil. BUT! Does that really justify the way she acted?
Do you guys remember the episode that was right before the Canterlot Wedding? It was Mmmystery of the Friendship Express; the one where Pinkie Pie's cake was ruined, and she made wild accusations about who did it. And who was the one who tried to get her to think with logic and get evidence? Twilight!
And who acted just as rashly as she did when confronted with this sort of issue? Twilight! Talk about practicing what you preach, right?
Okay, okay, in all honesty, despite Twilight being right overall, she wasn't technically right during the rehearsal; all she said was that Cadance was evil. She never said anything that suggested that "Cadance" was an imposter, a Changeling, or anything other than the Princess of Love. It's pretty clear that she really thought it was Cadance she was trash-talking! And with her egotistical, impulsive attitude about it, combined with all her claims being quickly debunked by her brother (as well as her previous feelings about being called to the wedding on such short notice), it's not so hard to see why no one would support her after she nearly wrecked the biggest event of Equestria.

3. Twilight's Friends (including Spike)
-Seeing that all of her friends had major duties to pull off for this wedding, it was easy to see why none of them would be focused enough to listen to Twilight's worries, especially combined with the fact that they could all see that she was already upset about the wedding before even reaching Canterlot. Plus, since they all know Twilight to be a usually logical friend (albeit, an impulsive one at times), they could've easily thought that Twi went nuts, especially if the episodes are in chronological order and saw a vast difference from her behavior in Mmmystery of the Friendship Express.
But, with that being said, I gotta admit that leaving her after their brother practically disowned her, and then just going on with the wedding without her, is a pretty big bitch move of them. One thing that I really wished to see was a perspective of what was going on at the castle between the rehearsal and the wedding. Did they try to visit her? Did they try to look for her? How long was the time between those two events? Any of those questions being answered could've easily affected how bad her friends really were during that time.
Either way, leaving a best friend after her brother is still pretty harsh, giant wedding duties or not.

4. Princess Celestia
-Honestly, this one pissed me off the most: not only did she leave Twilight without letting her say a word, AFTER her brother and all her friends ditched her (seriously, bad move Celestia!), and go on with the wedding without her. BUT! Remember what happened AFTER Canterlot was saved? What was the first thing she said to Twilight after the city was saved? Was it, "Thank you, Twilight?" Was it, "I'm sorry for ever doubting you?"
NOPE! All she says to her is, "You have a REAL wedding to plan!"

I'm sorry, but... she IS! She made no mention that she acted wrong in ANY way (despite not taking her most valued student's warning during a time of high threat), and just passed it off as a lesson for Twilight to not let doubt get to her. I mean, at least she said that the victory was as much Twilight's as her Brother's (which is true), but come on! If she never gave her the chance to get Alicorn wings or a castle, I bet Twilight would still be pissed!
Of course... In her defense, Twilight DID insult who she thought was her family during the rehearsal, and almost wreck a wedding Celestia probably had planned and financed for a while. That might make anyone ticked off, but still, it's the number one reason why I can't see Twilestia as a realistic shipping pair.

5. Queen Chrysalis
-Of course, if we have to be honest here, she really IS the biggest reason for all this happening. She's the master of deceit and manipulation, and was able to undermine everyone, with or without her spells! It was clear that she had her hooves in a lot of details to make sure that her victory could work, and it wouldn't surprise me if she was involved in a lot more behind the scenes.
Maybe she was the reason that Twilight and her friends had to make their plans in such a hurry! Considering that the Mane Six are huge national heroes, maybe she wanted to make sure that all of them would be busy and not notice anything was amiss. It wouldn't shock me to think that she was involved in Twilight not learning about the wedding until so soon to the ceremony.
Also, perhaps she played a part between the rehearsal and wedding as well. It's easy to assume that between the two events, Chrysalis made sure that none of Twilight's friends or Celestia would try to worry about her (maybe locking Twilight's room and leaving a note on the door, posing as Cadance or Twilight to assure them that things are fine, etc.). Since she organized a big invasion, it's not hard to believe that she took care of little details like that as well.
Overall, everyone screwed up big time, including Twilight, but none of it would've ever happened it if wasn't for Chrysalis. So, obviously, SHE deserves the most hate for the Royal Wedding fiasco.

How about you guys? Thoughts? Alternate ideas or opinions? Let me know below!

4220918 I didn't see Spike cause her any problems? He wasn't even there from what I see.

4220990 He was there, and he's just as guilty as everyone else. He has even less of an excuse than the mane six, since he was the only one who took Twilight's freak out in "Lesson Zero" seriously from the start.

4221238

Although you make a good point, let's not forget that Spike is still a kid (a baby dragon, even). It could be argued that Spike was too young to get the gravity of the situation, or maybe he left because he didn't want to get hurt as well. He still made a bad choice, no doubt, but I don't think that a child should be given as much blame for such a mature situation.

4222521 Kids are a lot smarter and more capable of dealing with things then you give them credit for. If Spike could handle Twilight's breakdown in "Lesson Zero" so maturely then he loses any and all excuse for not applying that lesson to Twilight's freak out in "A Canterlot Wedding".

4221238 Where was he? I didn't see him.

Personally I don't think Twilight is to blame, I know she didn't exactly help her case at the rehearsal and clearly she didn't think it through, but I am still very bitter towards the Princess and the others for what they did course I hate Chrysalis the most, like Tirek and the Dazzlings and Starlight, she makes my blood boil

4222712
Still it didn't look like he acted like the Mane 5 did to her though.
Looked more like peer pressure kept him quiet

So he shouldn't be blamed like the others

4220918
Spike shouldn't really be grouped with the Mane 5 in terms of blaming though.

4220918
I know this is late but I agree with a lot of what you said. Yeah Twilight was somewhat to blame for causing a ruckus and not gathering proof. Especially when you take into account she just solved a mystery from the last episode before the S2 finale. She should have investigated and gotten some real proof. Can't say whether Shining Armor was to blame or not but yeah all he saw was his sister throwing an apparent tantrum and making his fiance cry.

As for Celestia and the Mane 5 well I blame the bad writing when it came to the apology or lack of Apology being handled. Applejack's was really weak. Though when I wrote my first fanfic (ever) I came up with something to kind of explain why Celestia said the wedding thing (just for the sake of tying that loose end). I remembered Cadance can spread love and make couples forget they are arguing, so wouldn't the same result happen on a bigger scale when the love wave is unleashed? So the same thing could have happened with Twilight and the others, hence why Twilight is grinning so widely when the shockwave is unleashed.